Preheat the oven to 450°F. Place the tomatoes, garlic cloves and quartered onion on a baking sheet; drizzle with olive oil and season with salt. Roast for 30 minutes.
While the veggies roast, toast the chipotles in a small dry sauté pan for about 1 minute. Transfer a bowl and cover the boiling water to rehydrate. When the veggies are very well-browned and soft, remove from the oven and let cool to room temperature.