Preheat the oven to 200 ° (top and bottom heat). Do not pound the schnitzel too thin, season with salt, pepper and fry well on both sides. Then layer them in a small baking dish - you can also lay them on top of each other. Cut the onion into rings, fry until light brown and deglaze with milk. Stir in the balsamic vinegar and season with salt and pepper.
Pour the whole thing over the schnitzel so that each piece gets enough onion and bake in the oven for about 30 minutes - don`t worry if the onions get a little dark.