Steam sausages 8 to 10 minutes. Cool sausages until cool enough to handle. Slice at an angle and add into large frying pan. Saute sausage and onion chunks in oil and butter until the onions are softened and sausage and onions start to brown. Be sure to turn sausages to brown both sides. Add beer and mustard. Stir until mixture is well coated and warmed through. Pile into warmed rolls or serve with boiled potatoes, cooked cabbage and chunks of rye bread and butter. Serve with extra mustard for dipping.