Heat a grill or grill pan over medium-low heat.
In a food processor, pulse onion and garlic until finely chopped. Add 1 can black beans, cilantro, parsley, egg, and red pepper flakes and pulse to combine.
Transfer mixture to a large mixing bowl, add the remaining can of black beans and the bread crumbs. Season with salt and pepper, to taste, and mix until well combined.
Divide 2/3 mixture into 4 portions and form into patties. Save the remaining mixture for another use, like Black Bean Enchiladas. Place on hot oiled grill over medium-low heat and cook about 6 minutes a side or until heated through. Toast hamburger buns on a grill. Place a burger on the bottom of each bun. Top with lettuce, tomato and ketchup. Cover the burgers with the top of the bun and serve.