Start by removing the papery husks from each of the tomatillos. Then, rinse the tomatillos and chili pepper with water to remove any extra debris. Then, cut the tomatillos into small pieces and mince your onion and fresh cilantro.
Place your prepped tomatillos, chili pepper, and one tablespoon of onion in the pitcher of your blender with the garlic, one teaspoon of cilantro, lemon juice, water, and salt. Blend the salsa ingredients until the mixture is smooth and well combined.
Transfer the blended salsa to a serving dish and top with pieces of avocado, along with the remaining minced onion and cilantro.