Heat the oil over high heat in a heavy skillet. Add the bratwurst and cook them until they begin to brown. Sprinkle the onions around the sausage in the pan, turn the sausage, and stir the onions until both the sausage and onions are golden brown. Add the chicken stock and turn the heat down to a low simmer. Cover the pan and cook the sausage for 10 minutes. Stir in the apple butter, salt, pepper, and vinegar; cover the pan and cook the sausage for another 10 minutes. Serve with mashed potatoes.