Preheat oven to 500 degrees F. Cover a baking sheet (or two) with foil for easy cleanup.
Rinse and dry tomatillos and jalapenos. For a mild salsa, cut jalapenos in half, removing seeds and membranes. For a medium salsa, do that for just two of the jalapenos, leaving the other two whole. For a hotter salsa, leave all jalapenos whole. Spread peppers and tomatillos on baking sheet(s).
Peel onion and cut into large chunks (4-6 pieces, depending on size). Add onions and unpeeled garlic to the baking sheet(s).
Drizzle with olive oil, enough to lightly and evenly coat the tomatillos, peppers and onions. Toss if needed.
Bake for 15-20 minutes until tomatillos, peppers and onions are a little bit charred. Remove from the heat and allow to cool.
If you left any peppers whole, chop off the stem. Carefully squeeze the roasted garlic out of its skin.
Add all roasted vegetables to a blender, reserving anything that doesn’t fit and adding once it’s a bit blended. Be careful — sometimes warm liquids can expand when blended.
Once the vegetables are mostly blended, add cilantro, lime juice, salt and cumin. Continue to blend until your preferred consistency is reached (it should be mostly smooth). Taste and add more cilantro, salt or lime juice if needed.
Serve immediately or refrigerate in a sealed container for up to 5 days.