Make sauce: Saute onion in oil in a large frying pan over medium-high heat until softened, about 7 minutes. Add enchilada sauce, tomatoes, beans, oregano, chile and brown sugar. Cook, stirring occasionally, until hot and slightly reduced, 4 minutes. Stir in chicken and cook until it's hot, about 4 minutes.
Assemble nachos: Divide chips among bowls, top with chicken mixture, cheese, lettuce, and cilantro. Serve with lime wedges and hot sauce.