In a large nonstick skillet, cook onion in 1 tablespoon oil over medium heat until translucent but not browned, 2-4 minutes; remove from heat and set aside. In bowl of a food processor fitted with blade attachment, pulse half the beans and rice until mixture forms a thick paste. In a large bowl, add processed bean mixture, remaining black beans and rice, cooked onion, bread crumbs, egg yolks, 2 tablespoons salsa, salt and pepper; mix well with hands, squeezing until mixture holds together. Form bean mixture into 4 patties. Cook burgers over medium heat in remaining 1 tablespoon oil until firm and browned, 4-5 minutes on each side., In a small bowl, combine sour cream and remaining 1/4 cup salsa. Layer a lettuce leaf, burger, cheese and sour cream mixture on each bun bottom, adding tomato and red onion as desired. Replace bun tops.