Heat a large skillet over high. Add tomatoes, onion, jalapeno, and garlic; cook, turning occasionally, until vegetables are charred on all sides, 20 to 30 minutes (garlic will be done after 10 minutes).
Peel garlic. (For less heat, halve and seed jalapeno.) Cut tomatoes, onion, and jalapeno into chunks and place in a blender. Add remaining ingredients and blend until smooth.