Step 1: Preheat your oven broiler to high and line a baking sheet with aluminum foil.
Step 2: Place the serrano peppers, tomatillos, garlic cloves, and onion rounds on the baking sheet in a single layer.
Step 3: Broil for 5-7 minutes, turning occasionally, until the pepper skins are charred and the tomatillos are soft and blistered.
Step 4: Remove from the oven and let the vegetables cool for 5 minutes. Peel the roasted garlic cloves and discard the skins.
Step 5: Place the roasted peppers (remove stems), tomatillos, garlic, and onion into a blender or food processor.
Step 6: Add the chopped cilantro, lime juice, sea salt, and cumin to the blender.
Step 7: Pulse until you reach your desired consistency—either chunky or smooth.
Step 8: Taste and adjust salt or lime juice if necessary. Transfer to a glass jar and refrigerate for at least 1 hour before serving.