Preheat the oven to 500°F. Line a baking sheet with foil and spray with cooking spray. Place the tomatillos, onion, garlic, and peppers on the baking sheet. Roast 15-20 minutes, or until the tomatillos have charred slightly and are starting to release their juices. Remove from oven and let cool slightly.
After the peppers have cooled, carefully peel off the skin. It's a good idea to wear gloves to prevent getting spicy pepper juice on your hands and then inadvertently rubbing it in your eyes. If you want your salsa spicier, leave the pepper seeds in, or for a milder salsa scrape them out.
Add the tomatillos, peppers, onions, and garlic to a blender or food processor. Add the cilantro, lime juice, vinegar, salt, cumin, and red pepper flakes, and pulse until it's as chunky or smooth as you would like it. Add water as needed to thin to desired consistency.
Pour the mixture into a medium saucepan and heat to a boil. If you're planning to refrigerate the salsa, ladle into clean containers, then let cool. Use within one week.
Bring boiling-water canner, half full with water, to simmer. Wash jars and screw bands in hot soapy water; rinse with warm water. Pour boiling water over flat lids in saucepan off the heat. Let stand in hot water until ready to use. Drain jars well before filling.
To use this recipe for canning, start by doubling the recipe. After the salsa reaches a boil , ladle into sterilized jars, leaving 1/2 inch headspace. Add sterilized lids and bands. Process in a water bath canner for a minimum of 15 minutes, adding 5 minutes for each 2000 feet of altitude above sea level. Let cool, then store in a cool, dry place. Use within six months.