1 1/2 pounds ripe cherry tomatoes
3 tablespoons olive oil, divided
1 teaspoon chopped fresh thyme
3 garlic cloves, crushed through a press or thinly sliced
3 celery ribs, chopped
2 leeks (white and tender green), well-rinsed, chopped
2 medium white onions, peeled and chopped
1 tablespoon tomato paste
1/2 cup (lightly packed) basil leaves, stems reserved
1 (14-ounce) can diced tomatoes, with juice
1 cup water or vegetable stock
Salt to taste
Preheat oven to 500 degrees. Arrange cherry tomatoes on a baking sheet and drizzle with 1 1/2 tablespoons olive oil. Sprinkle with thyme and garlic, and toss to coat. Roast until tomatoes are softened but still hold their shape, 5 to 7 minutes. Set aside to cool.
Meanwhile, heat 1 tablespoon oil in a large, heavy-bottomed saucepan or soup pot over medium heat. Stir in the celery, leeks and onions. Cook, covered, until softened but not browned, 5 to 10 minutes. Stir in the tomato paste and the reserved basil stems.
Stir in chopped tomatoes with their juice, then add the cherry tomatoes. Pour in the water or stock, reduce heat to low and cook, uncovered, 20 minutes to blend flavors.
Press the soup through a coarse sieve or food mill, discarding the tomato skins and basil stems.
Return mixture to the same pot, adding salt to taste, and reheat gently.
Serve the soup hot in warmed bowls. Tear the basil leaves into pieces and scatter over each serving, then drizzle with remaining olive oil.