How to Make It
Whisk together sour cream, buttermilk, lime juice, adobo sauce, and 1 teaspoon each of the salt and cumin; set aside.
Heat oil in a large skillet over medium-high. Add onion, beef, chile powder, and remaining 1½ teaspoons salt and 1 teaspoon cumin; cook, stirring to break beef into pieces, until browned, about 6 minutes. Add beans; cook until heated through, about 3 minutes.
Arrange meat mixture, tomatoes, romaine, cheese, and avocado on a large platter. Drizzle with dressing, and sprinkle with tortilla chips.