First wash the pork tenderloin under running water and pat dry with kitchen paper. Any remaining fat on the fillet can be cut off. Then sear the fillet in a hot pan with oil on each side (approx. 1 minute per side). The fillet e.g., place in a baking dish and cook in a preheated oven at 100 ° C for at least 80 minutes. The meat is particularly tender due to the low temperature.
Cut the onions into small cubes, sauté in a large saucepan with a little butter and deglaze with white wine. Let the stock boil until it has boiled down to a third. Then add the vegetable stock and reduce to about half the volume again. Then add the cream and sour cream (or crème fraîche) and simmer over low heat. Then add the Monschau mustard and stir. Do not bring the sauce to the boil afterwards, but keep it warm on the switched off hotplate.
After the cooking time, take the pork tenderloin out of the oven and cut into medallions. Then season with salt and pepper, arrange the medallions with the side dishes and pour the sauce over them.