Peel the onions. Wash the fish. Boil 3/4 litres of salt water with 1 onion, lemon juice and mustard seeds. Add the fillets and let them simmer at low heat for about 15 minutes. Clean, wash and slice the zucchini, cut the remaining onions into rings.
Steam in 2 tablespoons of hot fat, season with salt and pepper. Heat the rest of the fat. Sweat flour in it and deglaze with milk. Add cheese and simmer for about 5 minutes, season. Wash dill and chop finely.
Take the fish out and let it drip off. Cover one fillet each with onions and zucchini, add some sauce and cover with a second fillet. Sprinkle with dill and garnish with lemon wedges.
This is accompanied by rice tossed in butter.