Clean, wash and drain the spinach. Wash and chop the dill. Wash the fillets, dab dry with kitchen paper and place them next to each other (light side down) on a board.
Season with salt and pepper. Halve the salmon slices. Put 1/2 slice of salmon on each fillet, sprinkle with dill. Roll up the fillets and stick them together with wooden skewers. Peel, wash and halve the potatoes and cook in boiling salted water for about 20 minutes.
In the meantime peel the onions. Cut one onion into slices, dice the rest finely. Peel garlic and chop finely. Boil up the onion slices, lemon juice, salt, peppercorns, sugar, bay leaf and 750 ml water in a wide, flat pot.
Carefully add the plaice rolls to the stock and let them simmer at low heat for about 10 minutes. Cook spinach in boiling salted water at low heat for about 2 minutes. Place on a sieve and drain well.
Heat the oil. Sauté onion cubes and garlic in it, add spinach and turn briefly. Season with salt and pepper. Heat 250 ml milk and 25 g butter. Drain potatoes, mash with a potato masher and stir in milk.
Season to taste with salt and nutmeg. Carefully remove the rolls from the stock. Pour the brew through a sieve. Heat 150 ml fish stock and remaining milk. Stir in sauce powder with a whisk and bring to the boil.
Cut the remaining butter into pieces. Whip underneath at low heat. Season with wine. Fill the mashed potatoes into the rim of an oven dish. Place spinach in the middle, place plaice rolls on top and pour the sauce over them.
Bake in the preheated oven (electric oven: 200 °C/ gas: level 3) for 12-15 minutes. Serve garnished with dill as desired.