Finely chop the onion and garlic. Wipe the mushrooms with a wet cloth and cut them into small pieces. Mix mushrooms, onion, garlic, mustard and egg. Add salt and pepper.

Cut the tenderloin into 4 medallions, salt and pepper. Heat the oil in a frying pan and fry the meat over high heat on both sides for 2 minutes. Put the filling on each slice and bake the medallions in the oven for 15 minutes at 180 degrees.