Peel and wash the potatoes, dab dry and grate coarsely. Clean, grate and slice the mushrooms. Fry the onion and garlic mixture in 1 tablespoon of hot oil. Fry the mushrooms in it.
Fry the grated potatoes while turning. Season with salt, pepper and thyme. Wash the fish, dab dry and divide into four portions. Sprinkle with lemon juice and season with salt and pepper. Place in a shallow casserole dish, add potato and mushroom mixture.
Bake in the preheated oven (electric oven: 200 °C/ gas: level 3) for approx. 20 minutes. Clean and wash the vegetables. Cut celery and carrots into thin strips, leek into narrow rings. Sauté vegetables in the remaining oil for about 3 minutes.
Dust with flour. Add stock and cream, bring to the boil while stirring and cook for 5 minutes. Season with salt and pepper. Arrange fish on vegetables on a plate. Garnish with fresh thyme leaves, parsley bouquet and lemon slice.