Drain the mushrooms. Melt the fat. Grind bread finely in the universal chopper. Peel and finely chop the onion. Fry onion cubes and grated bread in hot fat while turning until golden brown.
Add the frozen herbs and lemon juice and season to taste with a little salt and lemon pepper. Beat the egg whites until stiff and mix with the bread mixture. Put peas in 100 ml boiling water and boil up briefly.
Mix the crème-fraîche, cornstarch, a little salt and pepper and stir into the peas. Add the mushrooms and place in an ovenproof dish. Wash the fish fillet, dab dry and place on the vegetables.
Spread the crust mixture over the fish. Bake in the preheated oven (electric cooker: 225°C/ gas: level 4) for approx. 25 minutes. Meanwhile boil up 3/8 litres of water and a level teaspoon of salt.
Remove from heat and add cold milk. Stir in the purée flakes briefly. Stir again after one minute. Sprinkle with nutmeg and serve with the fish fillet.