How to Make It
Preheat the oven to 350°. On a large rimmed baking sheet, toss the tomatoes, onion and garlic with the oil and season with salt and pepper. Roast for about 40 minutes, until the tomatoes and onion are tender and browned in spots.
In a saucepan, bring the roasted tomatoes, onion and garlic and the remaining ingredients to a boil over moderately high heat. Cook over low heat, stirring often, until thick, about 30 minutes.
Scrape the mixture into a blender, add 1/2 cup of water and puree until smooth. Season the barbecue sauce with salt and pepper.