Combine Velveeta and drained tomatoes in 1-1/2-quart microwave-safe bowl; cover. Microwave on MEDIUM (50%) 2 minutes; stir. Continue microwaving on MEDIUM 2 minutes more or until Velveeta is melted. Add chicken and onions; mix well.
Spray large nonstick skillet with cooking spray; heat over medium-high heat. Place 1 tortilla in skillet. Spoon about 2/3 cup chicken mixture onto bottom half of tortilla; fold tortilla over to cover chicken mixture. Cook 1 minute on each side or until golden brown on both sides. Repeat with remaining tortillas and chicken mixture.