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 until tender, 25-30 minutes, 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.