Leave out the bacon while frying the onions. Add the mushrooms and sauté. Add parsley and garlic, as well as cream and sour cream. Boil.
Season the raw roast with salt and pepper and place in a baking dish or roasting pan. Pour the sauce over it and cover it in the fridge for 24 hours.
Then place the roast in the cold oven and cook it with a lid or covered with aluminum foil for about 2 hours at 180 degrees. (Each oven heats differently, so you may need to consider the empirical values with your own oven.)
Finally tie up the sauce if necessary.
Tip: In my experience, time and temperature are also right if you take heavier pieces of pork loin.