Wash the fish and pat dry with kitchen paper. Drizzle the fish fillets with the lemon juice and place in a baking dish. Sprinkle with the herbs and season with a little salt and pepper.
Melt the butter or margarine in a bowl in the microwave. Finely dice the onion and mushrooms and add to the butter. Then add the mustard, breadcrumbs and cheese to the bowl and mix well with the butter, then distribute evenly over the fish fillets.
Put the fish on the middle rack in the oven and cook in the hot oven for about 30 minutes until the surface is lightly browned.
For the puree, peel, wash and dice the potatoes. Cook in the stock in a saucepan for 10 minutes. Then add the peas and cream to the potatoes and simmer for another 5 minutes. Add a little butter, mash everything and season with salt, pepper and nutmeg.
For the cucumber salad, wash the cucumber and cut into thin slices. Make a salad dressing from all the other ingredients and season with salt, pepper and sugar.