COOK sausage in large skillet on medium heat 5 to 7 min. on each side or until browned on both sides. Remove sausage from skillet; cover to keep warm.
MELT butter in same skillet. Add onions; cook 3 min. or until tender, stirring frequently. Stir in mustard. Add sauerkraut; mix lightly. Cook 10 min., stirring occasionally.
TOP with sausage; cover. Cook an additional 5 min.