For the sauce, melt 2 tablespoons of fat, stir in flour and sweat for about 2 minutes. Remove the pot from the heat. Peel and chop 1 onion and celery. Clean the leek, cut into rings and wash.
Melt 1 tablespoon of fat. Add the vegetables and fry over medium heat for about 5 minutes, turning. Deglaze with Noilly Prat and top up with fish stock. Wash the basil. Season with basil, bay leaf and pepper.
Simmer for about 20 minutes. Peel the remaining onion and cut into fine cubes. Wash the fish, dab dry and season with salt and pepper. Put 1 tablespoon of oil and onion in an ovenproof dish.
Roll up the fillets loosely and place them in the tin. Pour on wine and cook in the preheated oven (electric cooker: 125 °C/ convection oven: not suitable/ gas: not suitable) for approx. 20 minutes. In the meantime, clean and trim the mushrooms and, depending on their size, chop them if necessary.
Wash parsley, shake dry, pluck leaves from the stalks and chop. Heat 2 tablespoons of oil. Add the mushrooms and braise for about 5 minutes, turning. Season to taste with salt and pepper. Fold in parsley.
Pour the stock through a fine sieve and collect it. Measure 400 ml and pour it quickly into the roux while stirring. Bring to the boil and simmer for about 2 minutes while stirring. Season with salt and pepper and refine with crème fraîche.
Arrange rolls on sauce and mushrooms.