Peel and wash the potatoes and cut them into sticks. Clean, wash and cut the leek into rings. Bring 200 ml broth to the boil and cook the potato pegs in it for about 10 minutes. Heat half-fat margarine in a pot.
Sauté the leek in it. Deglaze with 200 ml stock, bring to the boil and cook for about 5 minutes. Wash the fish, dab dry and cut into 4 pieces. Season with salt, pepper and lemon juice. Bring a little salted water to the boil in a frying pan and steam the fish for about 8 minutes with the lid closed.
Add finesse and processed cheese to the leek, stir. Season leek with salt and pepper. Chop parsley finely, except for something to garnish. Sprinkle potatoes with parsley, arrange leek and fish on plates.
Garnish with lemon zest and parsley.