Clean and wash the peppers and leek. Cut the pepper into strips, leek into rings. Steam both in 2-3 tbsp. hot clarified butter for about 10 minutes. Season with salt and pepper. Take out.
In the meantime peel and wash the potatoes. Heat 2 tablespoons of butter. Sauté 2 lightly heaped tablespoons of flour in it. Stir in milk, 200 ml water, cream and stock, bring to the boil. Simmer for about 5 minutes while stirring.
Stir in the mustard. Season the sauce with salt, pepper and 1 pinch of sugar.
Wash the fish fillets, dab dry and cut into 4 pieces. Season with salt and pepper and turn in flour. Heat 1-2 tablespoons of clarified butter in a large pan. Fry the fish for 1-2 minutes on each side.
Cover the potatoes and cook for about 20 minutes. Divide the vegetables into four greased ovenproof dishes. Place a piece of each type of fish on the vegetables. Pour mustard sauce evenly over the vegetables. (Or put everything in a large mould.).
Bake in a preheated oven (electric cooker: 200°C/circulating air: 175°C/gas: level 3) for approx. 15 minutes.
Wash the herbs and put a few stalks of chives aside. Finely chop the rest of the herbs. Drain potatoes and sprinkle with parsley. Take gratins out of the oven. Sprinkle with chives and garnish.
Add the parsley potatoes.