Clean the spinach and wash it thoroughly. Blanch in boiling salted water for about 1 minute and drain. Peel onions and cut into slices. Heat the oil in a pan and fry the onions briefly.
Add the spinach and season everything with salt, pepper and nutmeg. Wash, halve and add the tomatoes. Heat the fat in a pot and sauté the flour in it. Add stock and milk while stirring and let it boil well.
Season to taste with salt and pepper. Press the sausage mixture directly from the skin into the cannelloni. Place the spinach in an ovenproof dish, place the cannelloni on top and pour the sauce over them. Bake in the preheated oven (electric cooker 225°C/ gas: level 4) for approx. 40 minutes.
For the last 5-7 minutes, add the sliced mozzarella to the sauce and let it melt.