Preheat the oven to 450 degrees (roast option). On a rimmed baking sheet, place the tomatillos, onions, garlic and jalapeño - drizzle with a little oil. Roast in the oven for 10-12 minutes - the tomatillos will be soft and squishy, with the exterior being blistered.
Remove from the oven. Scrape the seeds out of the jalapeño, and squeeze the garlic out of the peel - add everything to the blender along with your cilantro and avocado. Pulse until combined.
Taste, then add salt - and pulse an additional time, adding lime juice as needed for flavor (totally optional).