Sort the spinach, wash and drain well. Line a baking tray with baking paper. Rinse fish, pat dry and season with salt and pepper. Place on the baking tray. Preheat the oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see
Peel and finely dice the onions. Peel, wash, coarsely grate and squeeze the potatoes. Mix potatoes, approx. 1⁄3 of the onions, mustard and crème fraîche. Spread the potato mixture evenly on the fish.
Bake in a hot oven for 20-25 minutes.
Meanwhile wash and halve the tomatoes. Heat the oil in a wide saucepan. Braise the tomatoes and the remaining onions briefly. Add the spinach and cover and let it collapse. Season tomato spinach with salt, pepper and nutmeg.
Remove the fish from the oven and serve with the vegetables.