In a medium bowl, mash the black beans with a fork until mostly smooth but still some whole beans remain.
Stir in diced red onion, breadcrumbs, cilantro (if using), cumin, chili powder, salt, and pepper. Mix well until combined.
Divide the mixture into two equal patties.
Heat olive oil in a large non-stick skillet over medium heat. Carefully place the patties in the skillet.
Cook for 5-6 minutes per side, or until golden brown and heated through.
Toast burger buns if desired.
Spread mashed avocado on both halves of the buns. Place a black bean patty on the bottom bun and top with lettuce, tomato, and red onion slices. Place the top bun on and serve.
