Peel the onion and clove of garlic, dice finely and sauté in 1 tablespoon butter. Add the frozen spinach and let it thaw over medium heat. When the potatoes are soft enough, mash them while adding the milk. Then mix with the spinach, season with pepper, salt and nutmeg and keep warm.
Now wash the pikeperch, pat dry, pepper and salt on both sides. Fry briefly in the second tablespoon of butter in a pan and keep warm as well.
Deglaze the roast with the cream and bring to the boil briefly. Capers, I like to add some more stock from the glass to the pan, as well as add lemon juice, then season with salt and pepper and serve.