Peel and finely chop the onion. Clean the spinach and wash thoroughly several times. Heat the fat in a pot, fry the onion until transparent and add the spinach. Let the spinach collapse and steam in an open pot for about 5 minutes until most of the liquid has evaporated.
Season spinach with salt, pepper and nutmeg and place in 4 ovenproof dishes. For the sauce, bring cream and stock to the boil. Stir in the sauce thickener and bring to the boil again briefly. Stir about 1 tablespoon of mustard into the sauce and season with salt and pepper.
Wash the dill, dab dry and chop. Stir 1/3 of the dill into the sauce. Wash the plaice fillets, dab dry and cut in half lengthwise. Place the fillets on a kitchen board with the skin side facing upwards. Sprinkle fillets with lemon juice, sprinkle with a little salt and spread a thin layer of the remaining mustard.
Then sprinkle it thickly with dill. Halve the salmon slices, place them on the fillets and roll them up. Fix with wooden skewers and distribute the rolls into the moulds. Pour sauce over them and bake in a preheated oven (electric cooker: 200°C/ gas: level 3) for about 25 minutes.
Serve immediately garnished with a slice of lemon.