Position a rack in the upper third of the oven, and heat broiler to high.
Place tomatoes and jalapeño peppers skin-side up on a large rimmed baking sheet with garlic cloves and onion. Broil until skin on tomatoes and peppers are blistered and charred, about five to seven minutes. Set aside to cool for five minutes. Remove as much of the charred skin as possible from the tomatoes and peppers. Remove skin from the garlic.
Place vegetables into a food processor with cilantro, lime juice, cumin and salt. Pulse until desired consistency. Serve warm or chilled.