Clean mushrooms. Peel the onions and cut both into slices.
Fry the mushrooms in heated oil for about 3 minutes. Add onion slices and sauté until translucent. Season with salt and pepper. Add the sauce and tomato paste and stir everything together. Mix the cream with cornstarch and stir into the sauce.
Layer the meat, sauce and bacon alternately in a flat baking dish. Finish with the sauce and bacon as the last layer.
Cook in the preheated oven (top / bottom heat: 175 ° C - 200 ° C, fan oven: 150 ° C - 175 ° C) for approx. 2 hours on the middle rack.
Note: After approx. 1 hour, cover the casserole with baking paper or aluminum foil.