Preheat the oven to 450 F (230 C) and set out a baking sheet. Option to lightly grease the baking sheet with avocado oil for easier cleanup.
To the baking sheet, add whole tomatillos (husks removed) and whole jalapeños. Bake for 15-20 minutes or until browned and bubbly, flipping with tongs halfway through. Remove from the oven and let cool slightly. Once cool enough to handle, remove and discard the jalapeño stems.
To a food processor (or high-speed blender), add chopped onion, garlic, cilantro, lime juice, salt, and roasted jalapeños and tomatillos. Blend until well combined. Taste and adjust as needed, adding more lime juice for acidity, jalapeño for heat, garlic for zing, cilantro for freshness, or salt for overall flavor.
Best when fresh as it will thicken after chilling (just stir and add a bit of water or lime juice as needed to thin). Store leftovers covered in the refrigerator for up to 4-5 days or in the freezer for up to 1 month.