01 - Combine ground flaxseed and water in a small bowl. Stir thoroughly and let stand for 5 minutes until thickened.
02 - Place drained black beans in a large bowl. Mash with a fork or potato masher, leaving some beans intact for texture.
03 - Add oats, diced onion, garlic, bell pepper, smoked paprika, cumin, chili powder, salt, pepper, cilantro, and prepared flax egg. Mix until thoroughly combined.
04 - Divide mixture into 4 equal portions. Shape each portion into a patty, pressing firmly to hold together.
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 - Whisk together vegan mayonnaise, chopped chipotle pepper, adobo sauce, lime juice, garlic powder, and salt in a small bowl until smooth.
07 - Lightly toast burger buns if desired for added texture and warmth.
08 - Spread chipotle lime mayo on both bun halves. Layer lettuce, tomato, onion, avocado, and the cooked black bean patty. Serve immediately.