Brown beef and onion in oil, stirring to crumble meat. Combine soup and milk in a saucepan and cook, stirring over medium heat until smooth. Then add chilies. Cut tortillas in 1-inch squares and make one layer in baking dish using half of cooked meat. Then half of soup mixture and half of cheese. Repeat layer. Bake 325 degrees for 30 minutes. Makes 6 servings.