Let peas thaw at room temperature. For the sauce hollandaise cook a reduction and clarify the butter.
Peel and finely dice the onions. Clean, wash and quarter the cabbage and cut into strips from the stalk. Heat oil and 1 tbsp. butter in a wide saucepan. Fry the onions in it until transparent. Add the cabbage and fry at low to medium heat for about 6 minutes, turning.
Add the peas and continue cooking for about 2 minutes. Season with salt, pepper and nutmeg. Keep warm.
Meanwhile, wash the dill, shake dry and cut finely. Rinse fish fillet, dab dry. Mix flour, 1⁄2 each TL salt and pepper in a deep plate. Turn the fish in it. Heat clarified butter in a large pan.
Fry the fish for about 2 minutes on the skin side first, then turn and continue to fry for about 2 minutes. Keep warm.
Finish the sauce hollandaise and season to taste with horseradish.
Arrange vegetables, fish and some hollandaise sauce, sprinkle with dill. Garnish with lemon slices if you like. Serve with the rest of the sauce. Boiled potatoes taste good with it.