In a large pot over medium-high heat, combine broth, water, chicken, pepper, and garlic powder. Bring to a boil, then reduce heat and simmer for about 20 minutes or until chicken is cooked through. Remove chicken, shred and return to broth. At this point, you may cover and refrigerate overnight.
In a large stock pot over medium heat, cook the onion and garlic in the olive oil until slightly browned, 3-4 minutes.
Stir in the salsa, diced tomatoes, tomato sauce, chili powder, corn, black beans, shredded chicken and the broth. (I like to swish about a half cup of water among the salsa and tomato cans/jars to get everything out and add that to the pot, too.)
Bring soup to a boil, and then reduce heat and simmer, uncovered, for as little as 30 minutes or up to two hours. I usually aim for an hour but have gone to either end of this range. Add salt and pepper to taste. (I add about 1/2 teaspoon salt and a few turns of the pepper mill)
Garnish as desired and enjoy!