In a large skillet, heat butter and oil over medium-high heat. Add sausage and cook, stirring occasionally, 5-6 minutes, or until browned. Remove sausage from pan with a slottted spoon, leaving as much oil in pan as possible. Add onion and peppers, and cook, 7-8 minutes or until tender.
Return sausage to pan; stir in beer and mustard. Bring to a boil, reduce heat, and simmer 10 minutes, stirring occasionally. Spoon sausage mixture evenly over bottom halves of rolls; top with sauerkraut and cover with top of rolls.