HEAT oil in medium saucepan on medium-high heat. Add onions, celery and garlic; cook 5 min. or until crisp-tender, stirring frequently. Stir in flour; cook and stir 1 min. Whisk in milk. Add potatoes, corn and thyme; cover. Simmer 20 min. or until potatoes are tender.
STIR in VELVEETA and mustard; cover. Simmer on medium-low heat 5 min. or until VELVEETA is melted. Stir in fish; cook 8 min. or until fish flakes easily with fork. Remove and discard thyme sprigs.
TOP chowder with bacon. Serve with crackers.