Place tomatillos into a large pot and cover with water; bring to a boil. Reduce heat to medium-low and simmer until tender, about 10 minutes. Drain.
Transfer tomatillos to a blender. Add avocados, sour cream, jalapeno pepper, lime juice, garlic, cilantro, fajita seasoning, and sea salt; blend until smooth.
Refrigerate blended sauce until flavors meld, at least 4 hours.