Cut the tomatoes roughly and soak them in cold water. Wash the potatoes and cook in plenty of boiling water for about 20 minutes. Rinse under cold water, drain and peel
Wash and clean the spinach thoroughly. Peel and finely chop the onion. Heat 1 tbsp. butter and fry the onion cubes until translucent. Add the spinach and let it collapse in a closed pot. Season to taste with salt and nutmeg
For the sauce, melt 2 tablespoons of butter in a saucepan. Stir in flour and sweat briefly. Add stock and milk while stirring. Bring to the boil, boil for 3-5 minutes. Roughly dice cheese, melt in the sauce and season with salt and pepper. Pour tomatoes into a sieve and add to the sauce
Cut the potatoes into slices. Squeeze the spinach lightly. Place the potatoes and spinach alternately in a greased casserole dish. Pour some sauce over each layer. Finish with potatoes. Crumble white bread. Spread white bread crumbs and 2 tablespoons of butter in flakes on the gratin. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 30-45 minutes