Heat the oil in a large skillet over medium-high heat. Add the onion, garlic, cumin, ½ teaspoon salt, and ¼ teaspoon pepper and cook, stirring frequently, until the onion has softened, 3 to 5 minutes.
Add the beans and ¼ cup water and cook, tossing occasionally, until warmed through, 2 to 3 minutes.
Top the lettuce with the bean mixture, queso fresco, avocado, salsa, and sour cream. Serve with the tortilla chips.