Cod tenderloin on a bed of beans with mussles

I had liked so much the main course at the Grand Atelier that I have tried to replicate the recipe.So, impress your guests with this delicious fish recipe!
BEANS
2/3 cup of white beans
1 small leek
Salt and pepper
MUSSLES
½ lb mussels
FISH
4 portions of Cod tenderloin (Thick filets)
1 Tbsp butter
Salt and pepper - I used piment d’Espelette instead of pepper
SAUCE:
1 shallot
1 Tbsp butter
1 tsp flour½ cup dry white wine (I use a Macon Villages)
½ cup crème fraiche (use heavy cream if you do not find crème fraiche)

Beans:
Cut the leek lengthwise in 2. Cook the beans in a pressure cooker with 3 cups of water and the leek. Season with salt and pepper. Our beans were probably last year’s crop and we had to cook them for 35 minutes.
Mussels:
Wash them until the water is clear, remove the algae if any, put in a pan, cover and cook over medium heat until they are open. Turn the heat off and keep warm.Fish: Melt the butter in a pan with a cover. Put the fish, add salt and pepper, cover and let cook over medium low heat, about 6 minutes)
Sauce:
Melt ½ of the butter in a saucepan. Add the chopped shallot. Drain the water from the mussels and add to the sauce. Add the wine. Add some of the water from the fish, if any. Add the cream. Season to taste. In a small bowl mix the other half of the butter and the flour until you have like a ball (beurre manié). Add to the boiling sauce, stirring constantly. Rectify seasoning.

Serve the sauce in a soup plate, add a few spoonfuls of beans, put the cod on top of the beans, and decorate around the cod with the mussels. Serve immediately.