Preheat a broiler to high, and place an oven rack on the top most setting.
Place tomatillos, jalapeños and garlic cloves on a rimmed baking sheet, and broil until tender and lightly golden. Some of the vegetables may finish before others, so take them off the baking sheet as needed. Timing will depend on your broiler, but this process took me about 20 minutes to complete.
Cut the stems off the jalapenos, and scrape out the seeds. Reserve the seeds. Peel the garlic. Add roasted tomatillos, jalapeños and garlic to the container of a blender. Chop the scallions up into big pieces, and add them to the blender along with the cilantro and lime juice. Puree all ingredients in the blender. Season to taste with kosher salt. If you’d like to increase the spiciness of the salsa, blend in some of the reserved jalapeño seeds.
Transfer the salsa to the refrigerator, and let chill before serving. Check seasoning once the salsa has chilled, and add more salt and lime juice if needed. Store in the refrigerator for up to 2 weeks.