Step 1: Preheat your oven broiler to high and place the oven rack in the top position, about 4-6 inches from the heat source.
Step 2: Arrange the cleaned tomatillos, peppers, and unpeeled garlic cloves on a parchment-lined baking sheet.
Step 3: Broil for 5 to 7 minutes, or until the tomatillo skins are blistered and blackened in spots. Flip them halfway through for even roasting.
Step 4: Remove the tray from the oven. Carefully peel the skins off the roasted garlic cloves and discard the skins.
Step 5: Transfer the roasted tomatillos, peppers, garlic, and any juices from the pan into a blender or food processor.
Step 6: Add the chopped onion, cilantro, lime juice, salt, and cumin. Pulse the mixture until it reaches your desired consistency—usually a slightly chunky texture is best for tomatillo recipes.
Step 7: Taste and adjust seasoning with more salt or lime juice if needed. Let the salsa cool to room temperature before serving to allow the flavors to meld.