Toast walnuts in a skillet over medium heat, stirring frequently until lightly browned and fragrant, 5 to 7 minutes. Crush into fine pieces using a mortar and pestle or food processor.
Heat olive oil in the same skillet over medium heat. Cook and stir onion until lightly browned, about 5 minutes. Season with salt and pepper.
Mash black beans in a large bowl using a fork. Add walnuts, onion, rice, barbeque sauce, cornflake crumbs, brown sugar, chile powder, paprika, and cumin. Season with salt and pepper; stir until mixture is well combined and holds together.
