Heat in a large skillet on medium heat; add onions, salt and pepper. Cook covered, 15 to 20 minutes or until onions are golden brown and soft, stirring occasionally. Remove from heat; keep warm. Grill sausage 8 to 10 minutes, turning frequently, until thoroughly heated. Remove from grill; keep warm. Spread mustard over cut surfaces of buns; place sausage pieces in buns. Top with onions and serve. Refrigerate leftovers.