Add the beef and other half onion in a non-stick skillet over medium-high heat until no longer pink and onions are translucent. Drain off the fat, and mix in the water and taco seasoning. Bring to a simmer, cover and turn down to low. Simmer for 8-10 minutes until tender. Stir in the black beans (add another 1/4 cup water if too thick) and simmer another 4-5 minutes.
Top with pico de gallo, avocados, extra cilantro, and sour cream. Serve immediately.