01 -
Set a cast iron skillet or comal over medium-high heat and let it get fully hot, about 2 to 3 minutes. No oil.
02 -
Add the tomatillos cut side down, whole serranos, jalapeños, garlic cloves, and onion wedges. Cook, turning occasionally, until everything blisters and darkens in spots, 8 to 12 minutes. The tomatillos should soften and turn army green. Remove each piece as it finishes.
03 -
Transfer the charred serranos and jalapeños to a zip-lock bag or a bowl covered with plastic wrap. Seal and let them steam for 5 minutes, then rub off and discard the loosened skins.
04 -
Squeeze the roasted garlic out of its papery skin. Roughly break apart the charred onion wedges.
05 -
Add the tomatillos, peeled chiles, garlic, onion, cilantro, lime juice, and salt to a blender. Blend on high for 30 to 45 seconds until smooth with a bit of texture. Taste and adjust salt and lime. Add water one tablespoon at a time if it needs loosening.
06 -
For a deeper, slightly less acidic flavor, pour the blended salsa into a small saucepan and simmer over medium heat for 5 minutes, stirring once or twice. Cool before serving or refrigerating.