Preheat oven to 400°. Place tomatoes, onion and garlic in a greased 15x10x1-in. baking pan; drizzle with oil. Sprinkle with thyme, salt and pepper; toss to coat. Roast 25-30 minutes or until tender, stirring once. Cool slightly.
Process tomato mixture and basil leaves in batches in a blender until smooth. Transfer to a large saucepan; heat through. If desired, top with croutons and sliced basil. Yield: 6 servings.