Peel and halve the onions. Clean and wash the mushrooms. Cut everything into slices. Finely dice the bacon. Pat the fillet dry.
Fry the meat in 1 tablespoon of oil in a large pan for about 5 minutes. Season, take out. Leave the bacon in the frying fat. Fry the onion and mushrooms.
Stir in 300 ml of water, cream and stock. Bring to the boil and simmer for about 5 minutes. Season to taste with salt, pepper and mustard, bind with sauce thickener.
Place the fillets in an oiled casserole dish or a large roasting pan (with a lid). Pour the sauce over them. Cover and simmer in the preheated oven (top / bottom heat: 175 ° C, fan oven: 150 ° C, gas: level 2) for approx. 45 minutes. Continue simmering after 30 minutes.
Wash, chop and sprinkle the parsley. It goes well with baguette and a salad.
Tip: You can fill the ingredients into the mold the day before and leave to marinate overnight in the refrigerator.