Heat olive oil in a saucepan over medium-low heat; cook onion in hot oil until tender, about 5 minutes. Stir black beans, green salsa, garlic powder, chili powder, and cumin with the onion. Reduce heat to low and cook the mixture at a simmer until it thickens, 5 to 10 minutes.
Serve with taco shells, Mexican cheese blend, tomato, avocado, and shredded lettuce.
