Sauté the diced onion in the butter and olive oil for 5 minutes, over medium heat.
In a large bowl, mix together the onion, sweet potatoes, black beans, and 2 teaspoons of the taco seasoning.
Dump the mixture into a large glass baking dish, cover tightly with foil, and bake for 50 minutes.
Combine the remaining 1 teaspoon of taco seasoning with the chicken, and evenly distribute it over the potato and bean mixture.
Recover the baking dish, and then bake for an additional 20 minutes, or until the chicken is cooked through.
Remove the dish from the oven, top with the parsley and cheese, and then recover for 5 to 10 minutes, or until the cheese has melted. There is no need to place the dish back in the oven, the steam and heat from the potatoes will melt the cheese.
Serve with sour cream, avocado, and olives if desired.