Combine flaxseed and water in a small bowl and let sit for 5 minutes.
Mash the beans in a large bowl with a fork. Add the breadcrumbs, ½ cup salsa, flaxseed mixture, garlic and onion powders, and cumin. Shape mixture into four equal sized patties and refrigerate for 30 minutes.
Heat oil in a large skillet over medium heat. Cook burgers for 5-7 minutes per side or until crispy on the outside and firm.
Spread top and bottom buns with mayonnaise. Serve on buns with remaining salsa, cilantro, and avocado slices.