Wash the fish fillet, dab dry and cut into portion pieces. Sprinkle with the juice of 1 lemon and let it stand for a while. Clean, wash and sort the spinach. Blanch in little boiling salted water for 1-2 minutes and let drain.
Bring 1/4 litre of water and milk to the boil and stir in the puree powder. Let stand for 1 minute, add butter and stir again. Season to taste with horseradish, salt and some cayenne pepper. Stir in spinach and keep warm.
Heat the oil in a frying pan. Dab fish fillets a little bit, season with salt, turn them in the flour and fry them on each side for 5-6 minutes until golden brown. Season with pepper. Wash remaining lemons and cut into thin slices.
Arrange the fish fillets on the lemon slices and serve with the spinach mashed potatoes. Sprinkle with some cayenne pepper and garnish with dill.