Wash the potatoes and bring to the boil covered with water. Cook potatoes for about 25 minutes, drain and peel. Clean and wash the spinach. Peel onion and garlic and chop finely. Heat oil in a pot.
Fry the onion and garlic in it. Add the spinach and let it collapse. Season with salt and pepper. Clean, wash and slice the tomatoes. Cut potatoes into slices. Layer spinach, potatoes and tomatoes in an ovenproof dish.
Drain the mozzarella, cut into slices and spread on the spinach. Bake in a preheated oven (electric cooker: 200 °C/ gas: level 3/ convection oven: not suitable) for approx. 10 minutes until the cheese has melted.
Serve the spinach casserole sprinkled with pepper.