Put a comal or pan over medium-high heat, add chiles and tomatoes, and let them roast until skins are smoky and start to break.
Turn heat off and let them cool down for 5 minutes.
Peel tomatoes and discard pepper tops. Put tomatoes, serrano peppers, cilantro, salt and onion in a blender. Using the ice crusher button on your blender, mix until blended but chunky. If your blender doesn't have an ice crusher button, just use the slowest speed and pulse it until mixed.