Place the tomatoes with juices in a baking dish and sprinkle with the dried thyme. Roast the tomatoes in a 350 degree oven for about 45 minutes. Stir often to evaporate most of the juice.
While the tomatoes are roasting heat the oil or butter in the soup pot. Add the onion and celery. Heat over medium-low stirring often, about 20 minutes. Optional step: Towards the end, add sherry and allow to cook down. Add in the stock.
Remove the tomatoes from the oven and add them to the soup pot along with the garlic. Cook the soup for about 45 minutes.
Pass the soup through a food mill (or blend w/ an immersion blender) and season with salt and pepper.
Optional: add a little cream or yogurt and heat without boiling.