Preheat your broiler to high heat. Cut the tomatillos in half and place them cut-side down on a baking sheet. Broil for 5-7 minutes, or until the tomatillos are charred and softened. Remove the tomatillos from the broiler and let them cool for a few minutes. Add the tomatillos, avocado, red onion, jalapeño pepper, garlic, cilantro leaves, lime juice, salt, and black pepper to a blender or food processor.