Preheat the oven to 170 ° C. Grease a baking dish with butter.
Finely chop the onions and spread them on the bottom of the baking dish, press the garlic. Cut the pork tenderloin into medallions, rub mustard and wrap with bacon. Place the medallions on the onions and garlic, cover with small flakes of butter. Put the pan in the oven for about 30 minutes.
Mash the Gorgonzola with a fork and mix with the sour cream, add the parsley, salt and pepper. If you like the sauce milder, you can leave out the salt. After 30 minutes of baking, pour the sauce over the meat and bake for another 15-20 minutes.