In anonreactive5- to 6-quart Dutch oven, heat the oil over medium-low heat. Add the onion, celery, and red pepper and cook, stirring occasionally, until the vegetables are quite soft, about 10 minutes. Add the broth, tomatoes, thyme, 1/4 tsp. each salt and pepper, and 1 cup water. Bring to a simmer over medium-high heat. Reduce the heat to low, cover, and simmer for 40 minutes.
Discard the thyme sprig. Let cool briefly and then purée the soup in two or three batches in a blender or food processor. Rinse the pot and return the soup to the pot. If it is too thick, add some of the reserved tomato juice.
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