File this under tailgate food, because our choose-your-heat-level salsa is going to go faster than a wide receiver. Roasting the tomatillos mellows their acidity and adds a subtle sweetness.
Position rack in upper third of oven; preheat broiler to high. Line a rimmed baking sheet with foil.
Rinse tomatillos under warm water and pat dry. Place the tomatillos, jalapeño(s) to taste and garlic on the prepared pan. Broil, flipping everything halfway through, until well charred, 8 to 10 minutes. Let cool for 5 minutes. Remove seeds from the jalapeño(s), if desired. Peel the garlic.
Transfer the vegetables to a food processor. Add cilantro, lime juice and salt. Pulse until the salsa is blended, but still has a chunky texture. Stir in onion before serving.
Refrigerate for up to 1 week.