Cut the liver, chicken, and onion into small pieces. Grind the liver separately (this is important!) with an immersion blender, then separately grind the chicken filet and onion. Mix everything and add the egg, pepper, and salt.
In a well-heated frying pan with pork fat or oil, spoon out the stuffing. Don't make the cutlets too big so they are easy to flip - they are delicate and can easily tear. Fry for 2-3 minutes on each side. Serve them with sour cream!