Heat olive oil over medium heat in a skillet; add onions and cook and stir until they begin to brown, about 5 minutes. Raise the heat to medium-high and add bell pepper, serrano pepper, garlic, ground cumin, ground turmeric, and cayenne pepper to onion, cook and stir until fragrant, about 1 minute.
Mix diced tomatoes, black beans, tomato sauce, green onion, lime juice, salt, and brown sugar into onion mixture; bring to a boil, reduce heat to low and simmer until flavors combine, about 30 minutes.
