Rinse the shrimps, dab dry and put aside about half of them. Cut the rest very small. Wash herbs, shake dry and chop finely. Mix prawns, herbs and 100 g butter. Season with salt, cayenne pepper and 1 tbsp. lemon juice, form into a roll with the help of a piece of cling film and chill.
Preheat oven (electric cooker: 80 °C/circulating air and gas: not suitable). Also heat a mould in it. Rinse fish, dab dry. Heat 2 tablespoons of butter in a small pot. Stir in 5 tbsp. lemon juice, 1 tsp. salt and pepper. Put the fish in the mould, spread the rest of the prawns around the fish and pour the butter mixture over it. Close the mould with aluminium foil. Cook in a hot oven for 30-40 minutes (core temperature 55-60 °C).
Peel and chop the onion. Heat the oil in a pot. Sauté onion in it. Add frozen spinach and 100 ml water. Cover and bring to the boil over medium heat, stirring several times. Simmer for another 2 minutes. Season with salt and pepper. Cut the shrimp butter into slices. 1-2 minutes before the end of the cooking time, remove the foil from the salmon and put butter on the salmon. Arrange everything. Boiled potatoes taste good with it.