Empty the salsa into a large saucepan. Cook 2 minutes over medium-high heat, then add the chicken, beans, broth, and cumin (if desired). Bring to a boil, lower heat to a simmer, and cook for 10 minutes, stirring occasionally. Top each bowl with a sprinkling of onions, a dollop of sour cream, and some tortilla chips (if desired). For a soupier dish, use 4 cups of broth.