Clean, wash and drain the spinach. Peel and finely chop the onion. Heat the oil in a pot and fry the onion in it. Add the spinach and let it collapse. Season with salt and pepper.
Peel and wash the potatoes and cut them into thin slices. Peel garlic and press it into the cream using a garlic press. Season with salt and pepper. Grease a gratin dish (approx. 23 cm Ø).
Squeeze out some of the spinach and layer it together with the potatoes in the mould. Pour spiced cream over it and cook in the preheated oven (electric cooker: 200 °C/ gas: level 3) for about 1 hour. Grate the Parmesan finely and spread it over the gratin about 20 minutes before the end of the cooking time and bake it au gratin.
Lamb chops and tomato salad are delicious with it.