Preheat oven to 500F.

Toss onion, (quartered lengthwise, with root intact), garlic cloves, chiles, tomatillos, with 1 tablespoon vegetable oil in a large bowl. Pour vegetables onto a foil lined baking sheet and spread into a single layer. TIP: Keep smaller vegetables (like the garlic cloves) towards the center of the tray where they are less likely to burn. Roast vegetables, turning once, until tender and charred, 15-20 minutes. Remove from oven and let cool slightly.

Once vegetables are cool enough to handle, transfer all the roasted vegetables (along with any juice collected on the baking sheet) and the cilantro into a food processor. If you like chunky salsa, pulse 4 or 5 times. For a smoother sauce, process longer (30-45 seconds); season with kosher salt, freshly ground black pepper, to taste.

DO AHEAD: Salsa verde can be made 5 days ahead. Cover and chill.

CREAMY VARIATION: Add 1/2 cup sour cream, 2 tablespoons lime juice, and the flesh of 1 ripe Haas avocado to the processor. Process until smooth and creamy.