Preheat your broiler to high. Remove the husks from the tomatillos and rinse well under water to remove the sticky residue.
Place the tomatillos, jalapeños, half the onion (cut into wedges), and unpeeled garlic cloves on a baking sheet. Broil about 4 inches from the heat for 8 minutes.
Flip everything over and broil for another 4–5 minutes, until the tomatillos are softened and charred in spots.
Remove from the oven and let cool for 10 minutes. Once the garlic is cool enough to handle, squeeze the cloves out of their skins.
Transfer the roasted tomatillos, jalapeños, onion, and garlic to a blender. Add the salt and cumin.
Blend until smooth, or leave it slightly chunky — your preference.
Dice the remaining half of the onion finely. Stir it into the blended salsa along with the cilantro and lime juice.
Taste and adjust salt, lime, or heat as needed.
Serve immediately or refrigerate. Keeps in the fridge for 5–7 days.