Peel and finely chop the onion. Peel the garlic and press it through a garlic press. Clean and wash the spinach and let it collapse in boiling water and drain well. Cook 2 lasagne plates each in boiling salted water for about 2 minutes and then place in cold water. Heat the oil in a pan. Fry the onions and garlic in it.
Deglaze with stock, cream and mascarpone and bring to the boil. Stir in the sauce thickener, bring to the boil again and stir in 3/4 of the parmesan. Season sauce with salt and pepper. Drain the lasagne sheets, cover each sheet with spinach, add some sauce and roll up. Fill the rolled lasagne plates into a greased ovenproof dish, spread the remaining sauce over them, sprinkle with the remaining parmesan and bake in a preheated oven (electric cooker: 200°C/ gas: level 3) for about 15 minutes