Peel and wash the potatoes and cook them in little boiling salted water for about 20 minutes. Peel onion and carrot. Halve onion and cut carrot into large pieces. Wash and slice the lemon.
Bring white wine, 3/8 litres of water, slices of lemon, onion, diced carrot, bay leaf, peppercorns, mustard seeds and salt to the boil in a pot. Simmer covered for 5 minutes. Clean, wash and dry the zucchini and cut them lengthwise into thin slices with a wide peeler.
Halve the salmon slices. Wash the plaice fillets and remove the two fillet halves from the skin with a sharp, thin knife. Cover each fillet with a strip of courgette and salmon, roll it up and fix it with wooden skewers.
Pour the stock through a sieve, reheat it and let the plaice fillets simmer for 5-6 minutes. Steam remaining zucchini slices in little boiling salted water and clarified butter for 2-3 minutes. Keep the plaice rolls warm.
Measure out 200 ml fish stock and bring to the boil with saffron. Mix crème fraîche and cornflour and thicken the stock. Bring to the boil again briefly and season with salt, pepper and Worcester sauce. Wash the dill, dab dry and chop finely, except for a little for the garnish.
Drain the potatoes, sprinkle with dill and toss briefly. Arrange plaice rolls with saffron sauce, courgette and potatoes on preheated plates. Serve garnished with a branch of dill.