01 - In a small bowl, combine ground flaxseed and water. Stir well and set aside for 5 minutes to thicken.
02 - In a large bowl, mash black beans with a fork or potato masher, leaving some texture intact.
03 - Add oats, diced onion, garlic, bell pepper, smoked paprika, cumin, chili powder, salt, pepper, cilantro if using, and the prepared flax egg to the mashed beans. Mix until well combined.
04 - Divide mixture into 4 equal portions and shape into burger patties of uniform thickness.
05 - Heat a non-stick skillet or grill pan over medium heat. Lightly oil if desired. Cook patties for 5 to 6 minutes per side until browned and heated through.
06 - In a small bowl, whisk together vegan mayonnaise, chipotle pepper, adobo sauce, lime juice, garlic powder, and salt until smooth and well incorporated.
07 - Toast burger buns if desired for added texture and warmth.
08 - Spread chipotle lime mayo on bun halves, layer with lettuce, tomato, onion, and avocado slices, then top with black bean patty. Serve immediately.