01 - In a small bowl, combine ground flaxseed and water. Stir and allow to sit for 5 minutes to thicken, forming a flax egg.
02 - In a large mixing bowl, thoroughly mash the drained and rinsed black beans with a fork or potato masher until they are mostly smooth, but still retain some texture for a satisfying bite.
03 - Add the breadcrumbs, finely chopped red onion, finely chopped bell pepper, minced garlic, tomato paste, the prepared flax egg, smoked paprika, ground cumin, chili powder, salt, and black pepper to the mashed black beans. Mix all ingredients thoroughly until uniformly combined.
04 - Divide the black bean mixture into 4 equal portions. Carefully shape each portion into a sturdy burger patty.
05 - Heat olive oil in a large skillet or non-stick frying pan over medium heat. Cook the formed patties for 4 to 5 minutes per side, until they develop a golden-brown crust and are heated through.
06 - While the patties are cooking, combine the vegan mayonnaise, minced chipotle pepper(s) in adobo sauce, adobo sauce, lime juice, garlic powder, and a pinch of salt in a small bowl. Stir well until the mayo is smooth and all ingredients are fully incorporated.
07 - Lightly toast the burger buns if desired, either in a toaster or on the skillet, for enhanced texture.
08 - To assemble, spread a generous amount of chipotle mayo on both halves of each burger bun. Layer with fresh lettuce leaves, a cooked black bean patty, sliced tomato, sliced red onion, and optional sliced avocado. Place the top bun half to complete the burger.