A potato and leek soup that tastes of the coast, because the butter and the mussels both put it there. Shallots and garlic are sweated in Bordier Seaweed Butter, then potatoes and leeks simmer in water for twenty-five minutes. Mussels are steamed open separately and their strained liquor goes into the pot, which is where most of the flavour comes from. The soup is blended smooth and the mussels go back in. Easy, and about an hour start to finish.
At a glance
- Difficulty: Easy
- Serves: 4
- Active time: About 25 minutes
- Cook: 25 minutes for the soup base, plus 3 to 4 minutes to open the mussels
- Key ingredient: 50 g (1/4 cup) Bordier Seaweed Butter
Why this works
There is no stock in this soup. It is built on water, and that is not an oversight. The flavour comes from two places: the seaweed butter the aromatics are sweated in, and the mussel liquor that goes in at the end. Bordier's seaweed butter dates to 1986, the first of the flavoured butters, made after a meal with friends by folding iodised Breton seaweed into hand-churned butter. Melted at the bottom of a soup pot it behaves like a marine base note, seasoning the potatoes from the first minute rather than sitting on top of the finished bowl.
The mussel liquor is the second half. A kilo of mussels gives up a surprising amount of concentrated, salty juice when they open, and strained into the blended soup it does the work a fish fumet would, without an hour of simmering bones. Cooking the mussels separately is the point: dropped straight into the pot they would overcook while the potatoes finished, and their grit would end up in the purée. Keep them apart, take their liquid, and add the meat only once the blender has stopped.
Ingredients
Use a floury baking potato so the soup blends smooth without cream. Bordier lists no stock, only water, which goes in during step one.
| Ingredient | Metric | US |
|---|---|---|
| Mussels | 1 kg | 2.2 lbs |
| Baking potatoes | 600 g | 1 1/3 lbs |
| Leeks, whites only | 2 | 2 |
| Bordier Seaweed Butter | 50 g | 1/4 cup (about 1 3/4 oz) |
| Shallots | 2 | 2 |
| Garlic | 1 clove | 1 clove |
| Bay leaf | 1 | 1 |
| Water (called for in the method) | 1 litre | About 4 1/4 cups |
| Salt and pepper | To taste | To taste |
Method
Step 1: Sweat the aromatics and simmer the soup base
Peel the potatoes and cut them into large chunks. Clean and slice the leeks. Peel and mince the garlic and shallots. Melt the seaweed butter in a pot, then sweat the shallots and garlic over low heat for 3 minutes without letting them colour. Add the potatoes, leeks and bay leaf. Pour in 1 litre of water. Salt, cover, and cook for 25 minutes over low heat.
Step 2: Open the mussels and take their liquor
Sort and rinse the mussels, discarding any with broken shells. Put them in a Dutch oven over high heat for 3 to 4 minutes to open, stirring them several times with a strainer. Let them cool. Set a few aside in their shells and shell the rest. Strain the mussel juice and add it to the soup.
Step 3: Blend and finish
Take out the bay leaf and blend the soup with an immersion blender. Add the shelled mussels. To serve, Bordier suggests a scattering of chopped parsley, croutons and the reserved mussels still in their shells.
Tips from our kitchen
- Strain the mussel liquor through cloth, not just a sieve. Even well-rinsed mussels release fine sand, and a sieve will not catch it. Line one with a coffee filter or a doubled square of muslin, and leave the last spoonful in the pan where the grit settles.
- Salt at the start, then again at the very end. The method has you salt the potatoes, which is right, but the mussel liquor arriving later is properly salty and the seaweed butter is salted too. Season lightly in step one and correct only after the liquor is in and the soup is blended.
- Sweat means no colour at all. Three minutes over low heat with the butter barely foaming. Browned shallots give a caramel sweetness that muddies the marine note this soup is built around.
- Blend the soup before the mussels go in. Mussel meat turns rubbery and stringy in a blender and will thread through the purée. Blend first, then stir the shelled mussels through the hot soup off the heat and let them warm for a minute.
- Baking potatoes, not waxy. A floury variety like russet breaks down and gives a velvety texture with nothing else added. Waxy potatoes go gluey when blended, which is the one failure mode this recipe has.
- Mussels that stay shut after 4 minutes should go in the bin, along with any that were already open and did not close when tapped before cooking. Give the pot a stir at the halfway point so the ones on top get their turn against the heat.
How to serve it
Serve it hot in deep bowls with the reserved mussels in their shells set on top, parsley and croutons over. Bread and more butter on the table is the Breton way to eat this, and a dry cider or a glass of Muscadet is the drink. If you want to understand why a flavoured butter changes a soup so much more than a spoonful of the same flavour added at the end, our guide to Bordier butter explains what the churning actually does.
Where to buy Bordier Seaweed 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 Seaweed Butter by Bordier (4.4 oz): hand-churned Breton butter with iodised seaweed, the base note that turns water and potatoes into a coastal soup.
For the other flavours, browse the Maison Bordier collection.
Frequently asked questions
How much Bordier Seaweed Butter do I need for this recipe?
You need 50 g, and a Bordier block is 125 g (4.4 oz). One block makes the soup twice with a little left for the bread you serve alongside.
Can I substitute a different butter?
Plain Bordier Salted Butter will make a good potato and mussel soup, but the seaweed is doing real work here since there is no stock in the pot. If you swap it, lean harder on the mussel liquor and hold back some of the water.
Can I make it without mussels?
Then it becomes a leek and potato soup with seaweed butter, which is a fine thing but a different recipe. The mussel liquor is the seasoning, so replacing it with a light fish stock is the closest substitution.
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. It picks up other smells readily. It also freezes for up to three months.
Does Bordier butter need refrigeration?
Yes. Bordier butter is a fresh, hand-churned product and ships cold in insulated overnight packaging. Unpack it into the fridge as soon as it arrives.
Can I make the soup ahead?
Make the blended base a day ahead and keep it in the fridge. Cook the mussels fresh on the day and add them, with their liquor, to the reheated soup. Mussels that have been reheated twice are not worth eating.
Related reading
- Bordier Butter: What It Is, Why Chefs Love It, and How to Use It
- La Brújula Spanish Conservas
- How to Choose Your French Butter: A Buyer's Guide
Recipe and photograph adapted from Maison Bordier, with thanks. The French Pantry is an authorized US retailer of Bordier butter.