Step 1 Heat the broiler. Arrange the tomatillos and jalapeno on a baking sheet and roast them under the broiler until their skins have begun to blacken and blister, 3 minutes. Turn them over and roast 2 more minutes. Remove from the oven. When cool, about 5 minutes, split the jalapeno and scrape away the seeds and skins.
Step 2 With the machine running, drop the garlic into the food processor feed tube and mince thoroughly. Stop the machine and scrape the tomatillos and any collected juices into the work bowl. Add the jalapeno and the onion. Pulse 3 or 4 times to make a coarse puree. Scrape down the sides of the work bowl as needed. The mixture should have a distinct body to it rather than being completely smooth. Stir in the chopped cilantro, add salt to taste and serve.