Remove the shell from the tomatillos and rinse well. Place in a large pot along with the peppers and cover with water 2/3 of the way up the pot. Bring to a boil then reduce heat to low. Simmer until tomatillos start to crack and split, roughly 20 minutes. Remove from heat and let cool.

Place cooked tomatillos and serranos in a blender along with all the cooking liquid. Add cilantro, garlic, onion and salt to taste. Blend until you’ve reached desired thickness.