Spread the tomatillos and peppers on a large cookie sheet. Roast under the broiler for about 5 minutes or until charred, shake and broil another 3 minutes. Remove from the oven and add to a large blender or food processor. Add the onion flakes and cilantro. Blend until mostly smooth. Season with salt and pepper to taste. Cool and serve. Can be stored in the refrigerator for 1 week or frozen for up to 6 months.