Heat a Griddle, Comal, or Carbon Steel Frying Pan over medium-high. Slice the half onion lengthwise into two quarters. Once hot, add serranos, tomatillos, and one quarter of the onion to the griddle. Cook, turning every few minutes, until the vegetables are softened and charred, 15–20 minutes, then set aside to let cool.

Add garlic along with a hefty pinch of salt to Molcajete and grind to a paste.

Once cooled, remove the stems (and seeds, if desired) from the chiles. Keeping them separate, roughly chop the vegetables.

Incorporate the charred onion into the garlic paste, smashing until fully combined. Repeat with the chiles. Once you have a uniform paste, add in the tomatillos a few pieces at a time.

Chop the remaining quarter of the onion and add to the Molcajete along with the cilantro, stirring until combined. Taste and adjust seasoning if needed. Yields about 1 ¼ cups.