Arrange tomatoes, onion, and garlic in an even layer on a jelly-roll pan coated with cooking spray. Sprinkle with 1/4 teaspoon salt and pepper. Broil 8 minutes; stir gently. Broil an additional 5 minutes or until vegetables begin to blacken at the edges.
Place tomato mixture and tomato paste in a blender or food processor; process until smooth. Place tomato mixture in a saucepan over medium heat. Stir in broth; bring to a simmer. Cook 10 minutes, stirring frequently. Remove from heat. Strain mixture through a sieve into a large bowl; discard solids. Return tomato mixture to saucepan over medium heat.
Combine 2 tablespoons water and cornstarch in a small bowl. Stir cornstarch mixture into tomato mixture. Increase heat to medium-high; bring tomato mixture to a boil. Cook for 1 minute, stirring constantly. Remove from heat.
Place fish on a broiler pan lightly coated with cooking spray. Sprinkle evenly with 1/8 teaspoon salt. Broil 6 minutes or until fish flakes easily when tested with a fork or until desired degree of doneness.
Place about 1/4 cup tomato mixture on each of 4 deep plates; top each with 1 salmon fillet. Sprinkle each with 1 tablespoon basil. Garnish with coarsely ground black pepper, if desired.