Salt and pepper the chicken schnitzel, fry well in the oil and then set aside.
Dice the onion and slice the mushrooms. Then put in the pan (in which the meat has already been seared) and fry. Then deglaze with the tomatoes and stir in the mustard. Place the schnitzel on top and cook for another 10 minutes with the lid closed.
Then take the schnitzel out again and pour the milk into the tomato sauce. Season the sauce with salt, pepper and parsley and serve with pasta or rice.