01 - Combine ground flaxseed with water in a small bowl and set aside for 5 minutes until thickened.
02 - In a large bowl, mash the black beans using a fork or potato masher, leaving some texture intact.
03 - Add diced onion, bell pepper, garlic, oats, breadcrumbs, flax egg, smoked paprika, cumin, chili powder, salt, and pepper to the mashed beans and mix thoroughly.
04 - Divide the mixture into 4 even portions and shape each into a burger patty.
05 - Heat olive oil in a large nonstick skillet over medium heat. Fry patties for 4 to 5 minutes per side until golden brown and warmed through.
06 - Combine vegan mayonnaise, minced chipotle peppers, adobo sauce, lime juice, and salt in a small bowl; mix until smooth.
07 - Optional: Lightly toast the burger buns to desired crispness.
08 - Spread chipotle mayo on burger buns, layer with lettuce, tomato, red onion, black bean patty, and avocado slices if using. Top with the remaining bun and serve immediately.