Peel and wash the potatoes and cook them in boiling salted water for about 25 minutes. In the meantime, wash the fish, dab dry and cut into 4 equally sized pieces. Season with salt and pepper. Whisk the egg in a deep plate.
Wash parsley, shake dry, chop finely.
Boil up the frozen vegetables and 8 tablespoons of water in a pot. Cover and cook over medium heat for about 6 minutes, stirring from time to time.
For the mustard sauce, heat 2 tablespoons of butter in a saucepan. Dust with flour, sweat lightly and add 1⁄2 l milk while stirring. Bring to the boil and simmer at low heat for about 5 minutes. Stir in mustard.
Season to taste with salt, pepper, 1-2 tablespoons of sugar and lemon juice.
Turn the fish first in egg, then in breadcrumbs. Heat clarified butter in a large frying pan and fry the fish in it at medium heat for 2-3 minutes on each side.
Drain the potatoes. Add 200 ml milk and 3 tbsp butter and mash everything with a potato masher. Season to taste with salt and nutmeg. Arrange everything and sprinkle with parsley.