01 - In a small bowl, combine ground flaxseed and water. Stir thoroughly and allow to sit for 5 minutes to thicken, forming your flax egg.
02 - In a large mixing bowl, thoroughly mash the drained black beans using a fork or a potato masher until they are mostly smooth, retaining some small chunks for desirable texture.
03 - To the mashed black beans, add the breadcrumbs, finely chopped red onion, bell pepper, minced garlic, tomato paste, the prepared flax egg, smoked paprika, ground cumin, chili powder, salt, and black pepper. Mix all ingredients until they are thoroughly combined.
04 - Divide the black bean mixture into 4 equal portions. Shape each portion into a firm burger patty.
05 - Heat the olive oil in a large skillet over medium heat. Carefully place the formed patties into the hot skillet and cook for 4 to 5 minutes per side, until they are golden brown and heated through.
06 - While the patties are cooking, prepare the chipotle mayonnaise. In a small bowl, combine the vegan mayonnaise, minced chipotle pepper(s), adobo sauce, lime juice, garlic powder, and a pinch of salt. Mix thoroughly until well blended.
07 - If desired, lightly toast the burger buns to your preference.
08 - Assemble the burgers: Spread a generous amount of chipotle mayonnaise on both halves of the toasted buns. Layer with fresh lettuce leaves, a cooked black bean patty, sliced tomato, sliced red onion, and optional sliced avocado. Crown with the other half of the bun and serve immediately.