Wash the pork loin, pat dry and cut into 3 cm thick medallions. Season the medallions with salt and pepper. Put 1 tablespoon of oil in the pan and heat. Fry the medallions vigorously on both sides. Reduce the heat, add butter and fry the meat for 2-3 minutes. Constantly drizzle with butter, remove, wrap in foil and keep warm.
Coarsely grind the peppercorns in the grinder, place in a sieve, briefly blanch in boiling water and drain. Sweat the peppercorns with half of the onions in the frying fat. Pour in the vegetable stock and cream, reduce to half over medium heat. Salt the sauce.
Clean mushrooms with a brush and cut into pieces. Heat 4 tablespoons of oil and fry the mushrooms in it. Add the remaining onion cubes and garlic and fry well while stirring. Add the white pepper sauce and the medallions together with the meat juice. Season to taste with salt and white pepper from the mill. Stir in parsley and serve.