Peel the onions. Clean mushrooms, wash if necessary. Dice bacon, mushrooms and onions. Fry bacon in a pan until crispy. Take out and fry onions and mushrooms.
Season with salt and pepper. Add bacon. Clean and wash the spinach and blanch in boiling salted water for one or two minutes. Drain the spinach. Chop 1/3 roughly and mix with the mushrooms. Fill the mixture into the cannelloni using a teaspoon.
Place the rest of the spinach in a flat, ovenproof dish. Place cannelloni on top. Heat the fat in a saucepan. Sauté flour in it and deglaze with cream and 1/4 litre of water. Season to taste with salt, pepper and nutmeg.
Finely grate the cheese. Melt half in the sauce and pour over the cannelloni. Sprinkle the rest of the cheese on top. Bake in the preheated oven (electric oven: 225 ° C/ gas: level 4) covered with aluminium foil for 30 minutes.
Remove foil and bake for another 15 minutes until golden brown.