Peel the onion and cut into strips. Fry in 1 tbsp. hot oil. Add frozen spinach and 200 ml water, bring to the boil. Season with salt and pepper. Simmer for 5-8 minutes.
Preheat oven (electric cooker: 200 °C/circulating air: 180 °C/gas: see manufacturer). Quarter Maultaschen. Fry in 2 tablespoons of hot oil for approx. 5 minutes until brown. Wash and dice the tomatoes and stir into the spinach with cream. Flavour with nutmeg.
Layer spinach and Maultaschen in a casserole dish. Grate the cheese on top. Bake in a hot oven for 12-15 minutes.