Peel and finely chop the onion. Heat the oil in a pot and sauté the onion. Add spinach and 3 tablespoons of water, cover and cook over medium heat for about 15 minutes. Crumble bread in the universal chopper. Peel garlic and dice finely.
Wash parsley, shake dry. Pluck off leaves and chop finely, except for a little bit for garnishing. Heat butter in a pan and toast bread in it until golden brown. Add garlic and parsley, remove from heat
Season spinach with salt, pepper and nutmeg to taste, if necessary drain a little. Wash and halve the tomatoes and put them in a casserole dish with the spinach. Pour cream over it. Wash the fish and dab dry. Season with salt and pepper and place on the spinach.
Mix Parmesan cheese into the breadcrumbs and season with pepper. Spread the breadcrumb mixture on the fish and press it down a little. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for about 20 minutes. Garnish with parsley