Soak the lentils in cold water for approx. 3 hours, then drain and simmer in a pan with water and bay leaf for approx. 25 minutes and drain then. Remove the bay leaf. It is quicker to use tinned lentils. Rinse them in a sieve and drain. Peel and finely dice the carrots, celery and 2 shallots. Sauté the vegetables in a pan in 1 tbsp oil. Add the lentils, cook briefly and deglaze with 50 ml white wine. Pour in the soup, stir in the tomato puree, 1 tbsp. vinegar and the parsley. Simmer until the liquid has almost boiled away. Season with salt and pepper.
Dice the other two shallots and bring to the boil in the remaining wine and vinegar. Reduce the sauce by a third. Stir both types of mustard into the sauce and season with salt and pepper. Season the fish fillets with salt and pepper and heat 2 tbsp. oil in a pan. Fry the fillets first on the skin side for approx. 5 minutes, add the butter, turn and continue frying. Arrange the lentil vegetables, sauce and fish on plates and season with a little coarse sea salt and pepper.