Cook and stir celery and onion in hot oil in large skillet until tender. Remove from skillet; set aside. Add meat to skillet; cook until browned, stirring occasionally. Drain. Add celery mixture to meat mixture in skillet; cook a little longer. Season with salt and pepper to taste. Add VELVEETA; cook until melted, stirring frequently. Stir in soup; simmer until thoroughly heated. Remove from heat; cool slightly. Spoon about 1/4 cup meat mixture onto each bun half; sprinkle with mozzarella cheese. Place on rack of broiler pan. Broil until golden brown. This doesn't take long so keep checking it. Makes 16 to 18.