In a food processor pulse the onion until chunky, about 10 times. Place onion in a strainer and rinse with water. Drain well and return to the food processor.
Add remaining ingredients and pulse until you reach your desired texture. This will take around 20 pulses, stopping now and then to scrape down the sides.
Choose tomatillos that feel firm with husks that completely cover the exterior.
Serrano chiles add a nice punch of heat. Leave the seeds and pith intact for the most heat, or remove them for a milder salsa. If preferred, you can substitute with jalapeno which isn't as spicy but still offers a little zip.
You can easily substitute it with white or yellow.
You will need 1 lime to yield 2 tablespoons, but you may want to grab 2 to be safe as each lime varies in its juice content. Fresh is best.