Preheat oven to 450°.
Reheat onion and pepper mixture in a microwave-safe bowl on high for about 2 minutes or until heated through, stirring once after about 1 minute.
In a separate dish, reheat sausages for 3 to 4 minutes or until heated through.
Slice each baguette quarter in half horizontally, cutting almost through to the other side. Arrange bread on a baking sheet, cut sides up. Top half of each portion with 1 ounce cheese. Bake at 450° for 3 minutes or until cheese melts and bread is lightly toasted. Remove from oven.
Spread mustard evenly over bread. Top with 2 sausage halves and 1/2 cup onion-pepper mixture.