Melt butter in medium skillet over medium heat. Add onion; cook 3 to 4 minutes or until onion is tender, stirring frequently. Add pork in barbecue sauce; cook 3 to 4 minutes or until thoroughly heated, stirring frequently.
Place hoagie buns, cut side up, on ungreased cookie sheet. Broil 4 to 6 inches from heat for 3 to 4 minutes or until toasted.
Spoon pork mixture onto bottom halves of toasted buns. Top with cheese slice. Cover with top halves of buns.