Halve the tomatoes and place them, cut sides up, in a pyrex baking pan. Drizzle with the olive oil. Sprinkle with a generous pinch of thyme, and scatter the garlic cloves in the pan. Sprinkle with salt.
Bake for 25 minutes. Lower the heat to 200°F. and cook another 2 hours. After 1 hour, drizzle with the vinegar. Baste occasionally. Remove from the oven, baste again, and let cool.