Heat the oven to 375°F and lightly grease a 9-by-13-inch baking dish.

Heat olive oil in a large skillet over medium heat. Add the diced onion and cook for 4–5 minutes, until softened, then add the minced garlic and cook for 30 seconds.

Stir in the black beans, pinto beans or vegetarian refried beans, corn, diced tomatoes with green chiles, salsa, taco seasoning, and water if needed.

Simmer the filling for 5–7 minutes, stirring occasionally. If using refried beans, continue stirring until the mixture is evenly combined.

Spread half of the tortilla chips or tortilla strips in the prepared baking dish. Spoon half of the bean mixture over the chips and sprinkle with about half of the cheese.

Repeat the layers with the remaining chips, bean mixture, and cheese. Scatter the sliced black olives over the top if using.

Cover the dish loosely with foil and bake for 15 minutes. Remove the foil and bake for another 10–15 minutes, until hot and bubbling and the cheese is melted.

Let the casserole rest for 5–10 minutes before serving. Garnish with green onions, cilantro, avocado, or other desired toppings.