A viennoise is a sheet of flavoured butter, breadcrumbs and cheese rolled paper thin and laid over fish before it bakes. It is a restaurant trick that costs almost nothing at home: the sheet melts into a crust while the cod underneath stays translucent and just set. Here it is made with Bordier Roscoff Onion Butter, and the same butter turns up again in a cream foam alongside a parsnip mash. Twenty minutes in the oven, but the crust needs 45 minutes chilling first, so start early.
At a glance
- Difficulty: Medium
- Serves: 2
- Active time: About 30 minutes, plus 45 minutes chilling for the crust
- Cook: 15 to 20 minutes at 180°C (350°F), depending on the thickness of the fish
- Key ingredient: 60 g (4 tbsp) Bordier Roscoff Onion Butter
Why this works
Bordier's Roscoff onion butter dates to 2016, made after Jean-Yves Bordier met the Johnnies of Roscoff, the Breton onion sellers who used to cycle their pink onions around Britain on handlebars. The AOP Roscoff onion is mild and faintly sweet, closer to a shallot than to a yellow onion, and folded into hand-churned butter it gives something between an allium and a caramel. That is why it can appear three times on one plate, in the crust, in the mash and in the foam, without the dish tasting repetitive. Each application pulls out a different part of it.
The viennoise itself is a piece of straightforward physics. Butter, breadcrumbs and parmesan rolled to 2 or 3 mm and chilled hard becomes a solid sheet you can cut to the exact footprint of the fillet. In the oven the butter melts down through the crumb and fries it in place while the parmesan browns and sets, so you get the texture of a fried coating without a pan, and the fish beneath cooks in the gentlest possible way. Chilling is not optional: a warm sheet tears when you lift it and slides off the fish as it bakes.
Ingredients
The 60 g of onion butter is split: 10 g goes into the foam and 50 g into the crust. Two Bordier butters appear here, since the parsnip mash takes a knob of plain salted butter.
| Ingredient | Metric | US |
|---|---|---|
| Cod fillets | 2 | 2 |
| Bordier Roscoff Onion Butter | 60 g | 4 tbsp |
| Bordier Salted Butter | 1 knob | 1 knob |
| Parmesan, grated | 50 g | 1/4 cup |
| Fine breadcrumbs | 50 g | 1/4 cup |
| Whipping or single cream | 30 cl | 1 1/3 cups |
| Fried onions | 2 tbsp | 2 tbsp |
| Parsnips | 2 | 2 |
| Curry powder | 2 pinches | 2 pinches |
| Olive oil | Quantity not specified | Quantity not specified |
Method
Step 1: Make the parsnip mash and the onion butter foam
Start with the mash. Cook the parsnips in salted water, then blend them with a little of the cream and the knob of salted butter. Now the foam: warm the rest of the cream in a pot with 10 g (2 tsp) of the onion butter and the fried onions. Let it infuse, then strain it and pour it into a siphon. Warm the siphon in a bain-marie just before serving.
Step 2: Salt the cod and roll the viennoise
Lay the two pieces of cod in a dish, salt them lightly to firm up the flesh, and refrigerate. Combine the breadcrumbs, 50 g (3 1/3 tbsp) of onion butter and the parmesan. Work the mixture smooth and shape it into a ball. Turn it out onto a sheet of parchment and lay a second sheet on top. Roll it out to a very thin rectangular sheet, 2 to 3 mm thick. Refrigerate for 45 minutes.
Step 3: Top the fish and bake
Once the sheet is properly chilled, cut bands the length of the cod fillets and lay one over each piece of fish. Bake at 180°C (350°F) for 15 to 20 minutes, depending on how thick the fillets are.
Tips from our kitchen
- Two ingredients are listed but never used. Bordier's list includes 2 pinches of curry powder and olive oil, and neither appears in the method. Our reading is that the curry belongs in the parsnip mash, where it is a classic pairing, and the olive oil is for greasing the baking dish. Both are our guidance, not Bordier's, so treat them as optional.
- Roll the sheet between parchment, then chill it flat. Slide the whole parchment sandwich onto a baking tray so it stays level in the fridge. A sheet chilled on a curve cracks when you try to cut bands from it.
- Salting the fish is a technique, not seasoning. A light salting draws surface moisture and firms the flesh, which stops the fillet weeping under the crust and pushing it off. Fifteen to twenty minutes in the fridge is enough, and pat the fillet dry before the band goes on.
- No siphon? Use an immersion blender. Infuse and strain the cream as written, warm it, then blend it at an angle at the surface so it draws in air and throws a head of foam. Spoon the foam off the top straight onto the plate. It is less stable than a siphon foam, so build the plate and serve immediately.
- Watch the crust, not the timer. Cod is done when the flakes just separate under gentle pressure and the centre is barely opaque. At 180°C a 3 cm fillet takes closer to 15 minutes than 20. If the crust is set but pale, give it 60 seconds under the broiler and no more.
- Use fine, dry breadcrumbs. Fresh crumbs hold too much moisture to roll into a clean sheet and the mixture will not hold its shape. Dry chapelure or panko blitzed fine both work.
How to serve it
Build the plate warm: a swipe of parsnip mash, the cod set on it with its crust intact, and the foam spooned or siphoned alongside rather than over the crust, which would soften it. Nothing green is strictly needed but a few leaves dressed in lemon lift the whole thing. A white Burgundy or a good Chablis suits it. If the Roscoff onion butter is new to you and you want to know where else it earns its place, our guide to Bordier butter runs through the range flavour by flavour.
Where to buy Bordier Roscoff Onion Butter in the US
The French Pantry ships Bordier butter across the United States in insulated overnight packaging, straight from the workshop in Brittany.
- Churned Roscoff Onion Butter by Bordier (4.4 oz): pink AOP Roscoff onion folded into hand-churned Breton butter, mild and faintly sweet, and the flavour running through all three parts of this plate.
- Churned Salted Butter by Bordier (4.4 oz): semi-salted at 2.8% fine salt, the knob that goes into the parsnip mash.
For the rest of the flavours, browse the Maison Bordier collection.
Frequently asked questions
How much Bordier butter do I need for this recipe?
The recipe uses 60 g of Roscoff onion butter, and a Bordier block is 125 g (4.4 oz), so one block covers it with about half left. You will also want a block of salted butter for the mash, of which you will use only a knob.
Can I substitute another butter for the Roscoff onion?
The nearest in spirit is Wild Garlic and Kampot Pepper Butter, which is savoury and allium-led, though sharper. Plain Salted Butter makes a perfectly good viennoise, but the fried onions in the foam then have to carry the onion character alone.
Can I use a different fish?
Any firm white fillet takes a viennoise well. Haddock, hake and pollock all behave like cod. Adjust the time for thickness and watch the flakes rather than the clock.
How do I store the butter once it is open?
Wrapped in its paper inside an airtight box, in the coldest part of the fridge, for about two weeks. Flavoured butter takes on other smells quickly. It freezes for up to three months, and the viennoise sheet itself freezes well too.
Does Bordier butter need refrigeration?
Yes. Bordier butter is a fresh, hand-churned product and ships cold in insulated overnight packaging. Get it into the fridge as soon as you unpack it.
Can I make the crust ahead?
Yes, and it is the easiest part to get ahead on. Roll the sheet up to two days in advance and keep it flat between its parchment in the fridge, or freeze it for a month and cut the bands from frozen.
Related reading
- Bordier Butter: What It Is, Why Chefs Love It, and How to Use It
- How to Choose Your French Butter: A Buyer's Guide
- Isigny Sainte-Mère: Normandy's AOP Butter & Crème Guide
Recipe and photograph adapted from Maison Bordier, with thanks. The French Pantry is an authorized US retailer of Bordier butter.