Preheat oven to 425°F. Rinse and dry the tomatillos. Then spread the tomatillos out evenly on a baking sheet , along with the serrano pepper. Roast in the oven for 15 minutes, or until the tomatillos start to brown on top and are cooked through. Remove from the oven, and transfer the tomatillos and serrano pepper to a blender or food processor. Add the garlic, cilantro, onion, lime juice, salt, and cumin to the blender or food processor, and pulse the mixture until it reaches your desired consistency. Taste and season with extra salt and/or cilantro if needed. Serve immediately, or refrigerate in a sealed container for up to 5 days.
