Turn on oven to high broil. (If you're oven has a broil temperature setting, set to 525 degrees). Line baking sheet with foil.
Peel off tomatillo outer shells and stems of serrano peppers.
Add tomatillos, peppers, and garlic to pan and broil until browned on tops and then rotate veggies and brown other side. About 6-7 minutes first side, 4 minutes second side.
Use a food processor (or blender) to blend all roasted veggies together.
Add diced onion, cilantro leaves, salt and red pepper flakes to food processor and blend until fully combined.
Serve while warm or chill if you prefer cold salsa. Flavor is great served both ways!
I personally love have some while it's warm and fresh and then save the other half to eat chilled!