Rub medallions with mustard and wrap with bacon. Spread half of the butter on an ovenproof dish, spread the chopped onion on the bottom. Now place the medallions on top. Spread the rest of the butter on top and put the mold in the preheated oven for 30 minutes at around 170 degrees.
Mash the Gorgonzola with a fork and mix with the sour cream, add the chopped parsley and green pepper.
Take the pan out of the oven, pour everything over the meat and then put it in the oven again for 10-15 minutes.
Note: The bacon and gorgonzola make it unnecessary to season the meat again. If you like, you can rub the casserole dish with garlic beforehand.