Puree tomatillos, garlic, chiles, and cilantro in a blender or food processor. Add the avocado and sour cream, processing just until combined.
Pour into container. Season to taste with salt, pepper, and lemon juice.
Rinse onion with cold water; pat dry. Stir diced onions and tomatoes gently into avocado mixture.
Please Note: Nutritional values listed may vary from final results depending on actual measurements and brands used to prepare the recipe.