1. Bring a saucepan of water to a boil. Cook tomatillos at a boil until just beginning to soften, 3 to 4 minutes; transfer tomatillos to a blender.
2. Combine jalapeno peppers, cilantro, onion, avocado, water, lime juice, and black pepper in the blender with the tomatillos; blend until smooth and creamy. Season with salt to serve.