Preheat broiler to high and line a baking sheet with foil.
Place tomatillos, jalapeƱos, onion quarters, and garlic cloves on the baking sheet and broil for 8-10 minutes, turning once, until charred and softened.
Remove from oven and let cool slightly, then peel the garlic cloves.
Add all roasted vegetables to a blender or food processor along with cilantro, lime juice, salt, and cumin.
Pulse until you reach your desired consistency, leaving it slightly chunky for texture.
Taste and adjust seasoning with additional salt or lime juice, then transfer to a serving bowl and let cool to room temperature before serving.