Shredded cheese, such as cheddar, Monterey Jack, or a Mexican blend
Sour cream(low fat or regular)
Preheat the oven to 350° F.
In a small bowl or plastic container mix together the onion powder, salt, chili powder, cornstarch, cumin, garlic powder, oregano, cumin, paprika, black pepper, and cayenne or red pepper flakes, if using. Blend well.
Spray a large skillet with nonstick spray, and place over medium high heat. Add the turkey, and cook, stirring and making sure to really break it up into small crumbles, until it is browned throughout, about 5 minutes. Drain off any liquid. Add the spice mixture and stir for 1 more minute so that you can smell all of the spices. Add 3/4 cup of water, and stir until the water is mostly evaporated and the meat is evenly coated with the spices, and there is a still a little bit of liquid in the pan.
Meanwhile, heat the taco shells on a baking sheet (or right on the rack, whichever you prefer) in the oven for 5 minutes until warm and toasty. Place any desired toppings into small individual bowls. Transfer the meat to a serving bowl, place the shells on a plate covered with a napkin or clean dishtowel to keep them warm, and set out with the bowls of toppings. Let everyone serve themselves.