Heat the oil in a large pot over medium heat. Cook and stir onion in hot oil until soft. Stir black beans, diced tomatoes, chicken, chili seasoning, cumin, salt and pepper through the onion, breaking up large pieces of chicken. Bring to a boil, reduce heat to medium low and cook at a simmer until hot, about 15 minutes.
Suggested toppings: shredded cheddar cheese, chopped fresh cilantro, sour cream, lime wedges, corn, sliced black olives, chopped red onions, tortilla chips, crushed corn chips.