PREHEAT flat top grill at 350°F. Add onions and oil; cook until onions are tender and lightly browned, stirring occasionally. Add meat; cook until thoroughly heated, stirring occasionally.
FOR each serving: Place roll halves, cut sides down, on grill; cook until lightly toasted. Fill with 3/4 cup of the meat mixture; drizzle with 3 Tbsp. of the sauce. Serve immediately.