Heat 1 tablespoon of butter and 1 tablespoon of oil in a heavy skillet over moderate heat. When the foam subsides, add the onions and cook for 3 to 5 minutes or until transparent. Remove to a small bowl and set aside. Add the remaining butter and oil to the skillet and when the foam subsides, add the veal scallops. Fry them over moderate heat until they are light golden brown, 4 to 5 minutes on each side. Remove to a heated platter and keep warm in a 220�F oven while you make the sauce. Pour off all but a thin film of fat from the skillet and add the cooked onions. Cook over high heat stirring constantly for 2 to 5 minutes. Lower heat and stir in sour cream and cheese a little at a time. Continue stirring until the cheese has melted and the sauce is smooth; do NOT allow it to come to a boil. Taste for seasoning and return veal to the skillet. Baste the meat with sauce and let simmer uncovered for 1 or 2 minutes. Serve immediately.