Peel onion and cut into rings. Grease an ovenproof pan or flat roasting pan and spread onions in it. Wash the meat, dab dry and wrap a slice of bacon around each.
Spread mustard on each side and spread on the onions. Cook in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 40 minutes. For the sauce, heat up cream, cream cheese, processed cheese and pepper.
Wash the parsley, shake dry, pluck the leaves from the stalks, chop and add to the sauce, except for a little garnish. 10 minutes before the end of cooking time, pour the cheese sauce over the medallions. Sprinkle with parsley.