Wash the potatoes and cook in boiling water for about 25 minutes. Clean and wash the spinach. Peel and finely chop onion and garlic. Heat oil in a pot. Brown onion and garlic in it.
Add the spinach and steam for 4 minutes. Season with salt and pepper. Clean and wash the tomatoes and cut them into small cubes. Cut cheese into 12 slices. Drain potatoes, rinse, peel and halve.
Place 3 potato halves in 4 greased casseroles. Spread spinach on top and sprinkle with tomato cubes. Put cheese on the spinach potatoes and bake in the preheated oven (E: stove 200 °C/ gas: level 3) for about 10 minutes.