Thaw the spinach, sauté in the heated garlic butter, season with salt and nutmeg. Preheat the oven to 160 ° C. Rinse the fish and carefully pat dry, season with the lemon juice, pesto and a little salt. Place the fish in a medium-sized baking dish, spread the spinach on the fillets. Drain the mozzarella, cut into slices and place on top of the spinach. Let the casserole cook in the oven for about 20 minutes.