Wash the potatoes and cook in boiling water for about 15 minutes. Quench potatoes cold, peel and let them cool down. Peel and chop onion. Fry in hot oil until translucent. Add spinach and defrost at low heat while stirring occasionally. Season with salt and pepper.
Wash, clean and slice the tomatoes. Melt fat, add flour and sweat briefly while stirring. Add tomato juice and milk bit by bit. Season with salt, pepper and nutmeg. Grate cheese. Cut potatoes into slices. Layer potatoes and spinach in an ovenproof dish. Sprinkle individual layers with 2/3 of the cheese, season with salt and pepper. Add some sauce to each layer. Cover the casserole with tomato slices. Spread the rest of the cheese on top.
Cut potatoes into slices. Layer potatoes and spinach in an ovenproof dish. Sprinkle individual layers with 2/3 of the cheese, season with salt and pepper. Add some sauce to each layer. Cover the casserole with tomato slices. Spread the rest of the cheese on top. Bake in the preheated oven (electric oven: 200 °C/ gas: level 3) for 35-45 minutes. Garnish with basil leaves as desired