Preheat oven to 450°F.
Remove the husks and stems from the tomatillos and then rinse and scrub them thoroughly to remove the sticky coating.
Trim the stems from the peppers, cut them in half lengthwise, and scoop out all of the seeds.
Trim the ends from the onion, peel the outer layer of skin, and then cut it into quarters.
Place the tomatillos stem-side up and jalapeño peppers skin-side up onto a large baking sheet along with the quartered onion and garlic cloves.
Drizzle oil over all of the vegetables and rub it into their skin.
Roast the vegetables in the preheated oven for 20-25 minutes, or until the tomatillo skins begin to brown. Pull the garlic cloves out after 15 minutes to prevent burning.
Add the roasted tomatillos, onion, jalapeño, garlic cloves that have been peeled, and any oil on the baking sheet to a large food processor along with the remaining ingredients.
Pulse ingredients 10-20 times and then scrape down the sides of the bowl. Continue pulsing until the salsa reaches your desired consistency. (About 40-50 times.)
Serve immediately or store in the refrigerator for up to 24 hours for an even better flavor. Salsa will keep well for up to 4-6 days when stored in an airtight container in the refrigerator or up to 2-3 months if frozen.