Place the tomatoes, garlic, and jalapeños on a foil-lined baking sheet. Broil 16 minutes, turning after after 8 minutes. Cool and peel tomatoes and garlic. Combine the garlic and peppers in a molcajete, mortar, or bowl; pound with a pestle or the back of a spoon to form a paste. Add tomatoes, and coarsely crush using the pestle or spoon. Combine the tomato mixture, cilantro, and remaining ingredients in a small bowl.
Roasting tomatoes gives them a sweet and complex taste; rinsing onion under cold water makes it crisp; and crushing roasted garlic and chiles in a mortar releases their full flavor. Though it's typically done in a molcajete (Mexican lava rock mortar), you can also make this salsa using a marble mortar and pestle (as we did). Finely chopping the garlic and chile in a food processor then adding the tomatoes and pulsing the mixture until everything is coarsely pureed will yield great-tasting results as well.