Serves 6:
600 g haddock, 1 kg Bouchot mussels, 600 g leaf spinach, 2 shallots, 60 cl milk, 30 g butter, 2 tbsp dry white wine, thyme, bay leaf, salt, ground pepper.
For the sauce: 200 g butter, 2 shallots, 10 cl vinegar, 15 cl dry white wine.
Soak the haddock in milk for 2 hours. Drain, gently squeeze, remove skin and bones, then cut into cubes.
Clean the mussels by scraping off any barnacles and removing the beards. Rinse thoroughly under cold water.
Peel and finely chop the shallots.
In a saucepan, combine the white wine, thyme, bay leaf, chopped shallots, and mussels. Cover partially and cook until the mussels open.
Drain the mussels, remove from shells, and set aside.
Prepare the spinach: remove stems, wash in several changes of water, drain well, then wilt in butter in a skillet. Season with salt and pepper.
For the sauce, finely chop the two remaining shallots and place in a sauté pan with vinegar and white wine. Reduce until only about a spoonful of liquid remains.
Cut the butter into small pieces and whisk it in gradually off the heat to emulsify. Season with salt and pepper.
Add the shelled mussels and haddock cubes to the sauce. Gently reheat without boiling.
Arrange the wilted spinach in individual casserole dishes, then top with the mussels and haddock mixture.
