Peel and finely chop the onion. Heat 1 tablespoon of oil in a pot. Sauté onion in it. Add the frozen spinach and 2 tbsp. water, steam over medium heat for about 15 minutes.
Rinse fish, pat dry, dice. Heat 1 tablespoon of oil in a frying pan. Fry the fish in it all around for about 4 minutes. Season with salt and pepper and take out. Heat butter in the frying fat. Fry the gnocchi for 6-8 minutes until golden brown.
Season with salt and pepper. Stir the lemon zest, juice and cheese into the spinach. Season to taste with salt and nutmeg. Mix in the fish and gnocchi and warm up again briefly.