Place fish on rack of broiler pan sprayed with cooking spray.
Broil 5 to 6 min. on each side or until fish flakes easily with fork, brushing frequently with melted butter. Cook and stir celery, onions and peppers in remaining butter in medium skillet on medium heat 3 to 5 min. or until crisp-tender. Stir in flour until blended.
Add tomatoes and VELVEETA; cook on low heat until VELVEETA is completely melted, stirring occasionally.