Cover the potatoes with their skins and cook for about 20 minutes. Then quench, peel and let cool down a little.
Peel and finely chop the onion. Heat 1 tablespoon of oil in a pot. Sauté onion in it. Add frozen spinach and 5-6 tbsp. water. Cover and steam over medium heat for about 15 minutes, stirring occasionally.
Then continue steaming open for about 5 minutes. Season to taste with salt and nutmeg.
For the sauce, heat 2 tablespoons of butter in a saucepan. Dust with flour and briefly sauté lightly. 1⁄4 Stir in l water and milk. Bring to the boil and simmer for about 5 minutes. Cut cheese into pieces and melt in the sauce.
Chop the tomatoes into small pieces, add. Season sauce with salt and pepper.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Grease an ovenproof dish. Cut potatoes into slices. Place potatoes and spinach alternately in the dish, pouring a little sauce over each layer.
Finish with potatoes. Crumble bread. Mix with 2 tablespoons of butter and spread in flakes on the gratin. Bake in a hot oven for about 30 minutes until golden brown.