Cook the potatoes in boiling water for 20 minutes. Clean and wash the carrots and leek. Cut leek into rings. Cut carrots into large sticks. Cook the carrots in boiling salted water for 10 minutes.
Add the leek 5 minutes before the end of the cooking time. Peel the potatoes and let them cool down a little. Wash fish, cut into 4 pieces and sprinkle with lemon juice, salt and pepper. Boil up broth, milk, cream and mustard.
Stir in the sauce thickener, bring to the boil again. Season to taste with salt, pepper and sugar. Cut potatoes into slices. Heat oil in a pan and fry the potatoes until golden brown. Put the vegetables, fish and potatoes in an ovenproof dish.
Pour gravy over it. Bake in the preheated oven (electric: 200 °C/ gas: level 3) for 25 minutes.