01 - Place the drained and rinsed black beans in a large mixing bowl. Using a fork or potato masher, mash the beans until they reach a coarse texture with some chunks remaining — avoid turning them into a smooth paste.
02 - Add the grated carrots, chopped mushrooms, diced red onion, corn kernels, minced garlic, chopped parsley, rolled oats, breadcrumbs, smoked paprika, ground cumin, salt, and black pepper to the bowl with the mashed beans.
03 - Crack the egg into the bowl (or add the prepared flax egg for a vegan version). Stir everything together thoroughly until the mixture is evenly combined and holds together when pressed.
04 - Divide the mixture into 4 equal portions. Using your hands, shape each portion into a patty approximately 1 inch thick, pressing firmly so they hold their shape.
05 - Heat the olive oil in a large skillet over medium heat. Carefully place the patties in the pan and cook for 4 to 5 minutes per side, until both sides are golden brown and the patties are cooked through.
06 - While the patties rest, toast the burger buns in the skillet or under a broiler until lightly golden on the cut sides.
07 - Place each patty on a toasted bun and top with lettuce leaves, tomato slices, pickles, ketchup, or any toppings you prefer. Serve immediately.