Heat oven to 450°F. Place bread halves, cut sides up, on ungreased cookie sheet.
In small bowl, mix butter and garlic. Spread mixture over cut sides of bread. In medium bowl, mix pork and barbecue sauce. Spoon pork evenly over bread. In large bowl, mix cheese, mayonnaise and 1/4 cup of the onions. Spoon dollops of cheese mixture over pork.
Bake 10 to 12 minutes or until cheese is melted and bread is lightly browned. Cool 5 minutes. Sprinkle with remaining 1/4 cup onions. Cut each bread half into 10 pieces. Serve warm.