01 - Combine ground flaxseed and water in a small bowl. Let sit for 5 minutes until thickened.
02 - In a large bowl, mash black beans with a fork or potato masher, keeping some texture.
03 - Add diced onion, bell pepper, minced garlic, rolled oats, breadcrumbs, flax egg, smoked paprika, cumin, chili powder, salt, and black pepper to the beans. Mix thoroughly.
04 - Divide mixture into four equal portions and shape each into a patty.
05 - Heat olive oil in a large nonstick skillet over medium heat. Cook patties 4 to 5 minutes per side until golden brown and heated through.
06 - In a small bowl, combine vegan mayonnaise, minced chipotle peppers, adobo sauce, lime juice, and salt. Mix well.
07 - Optionally, toast burger buns to desired crispness.
08 - Spread chipotle mayo on buns, layer with lettuce, tomato, red onion, black bean patty, and avocado slices. Top with the remaining bun and serve immediately.