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. (If you want less heat, cut the jalapeno in half before adding and remove the seeds.) Add the cilantro, onion, lime juice, and sea salt. Pulse until the desired consistency is reached.
Taste and add more salt to your liking if needed.