Preheat the oven to 450 degrees F (232 degrees C). Line a baking sheet with foil.
Arrange the tomatillos, jalapeno, and garlic in a single layer on the baking sheet. Drizzle with olive oil. Roast for about 10 minutes, until tomatillos are browned on the bottom. Flip and roast for 5 to 10 more minutes, until tomatillos are soft, browned on the bottom again, and skins are puckered.
Set the pan aside to cool completely to room temperature. (This is optional; you can also just refrigerate the salsa after making it.)
Once cooled, transfer the tomatillos, jalapeno (remove the stem first!), and garlic into a food processor. Add the avocado, cilantro, onion, lime juice, and sea salt. Pulse until the desired consistency is reached.
Taste and add more salt to your liking if needed.