Peel and finely chop the onion. Heat the oil in a pot. Sauté the onion in it. Add the frozen spinach and 3 tbsp. water and cook covered for about 15 minutes. Continue cooking for approx. 5 minutes open, so that the cooking water evaporates.
For the crust, crumble the bread finely. Peel garlic and dice finely. Wash parsley, shake dry and chop finely. Heat butter in a pan. Roast the bread in it until golden brown. Add garlic and parsley, roast briefly and remove from heat.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Wash and halve the tomatoes. Season spinach with salt, pepper and nutmeg. Spread with tomatoes in an ovenproof dish. Pour cream over it.
Rinse salmon fillet, dab dry and cut into 4 slices. Season with salt and pepper and place on the spinach. Grate parmesan and mix with the breadcrumb mix. Spread on the salmon and press on a little.
Bake in a hot oven for about 20 minutes. Mashed potatoes go well with it.