For the best flavor use fresh, ripe tomatoes. Roasting brings out the natural sweetness in the tomato and the onion. Good served hot or cold.
Preheat oven to 400°F. Spray a large rimmed baking sheet with nonstick cooking spray. Place tomatoes, onion, and garlic on the prepared pan. Drizzle with the olive oil, thyme, 1/2 teaspoon salt, and pepper. Shake the pan back and forth a few times to coat the vegetables with the oil and seasonings. Bake until tender, about 25 minutes. When cool, blend the roasted tomatoes, along with the basil and the remaining 1/2 teaspoon salt, in batches if necessary. Transfer to a saucepan and heat if desired, or refrigerate to chill.