For the taco filling:
Preheat the oven to 350 degrees F. Place the chili powder, cumin, onion powder, salt, cornstarch, garlic powder, oregano, paprika, black pepper, and cayenne or red pepper flakes, if using, in a small bowl or plastic container. Blend well. Spray a large skillet with nonstick cooking spray and place it over medium-high heat. Add the turkey or beef 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 cook stirring, until you can smell all of the spices, about 1 minute. Add 3/4 cup of water and stir until the water is mostly evaporated, the meat is evenly coated with the spices, and there is still a little bit of liquid in the pan, about 4 minutes. To serve, place your choice of toppings in small individual bowls. Heat the taco shells on a baking sheet (or right on the oven rack, whichever you prefer) in the oven until warm and toasty, about 5 minutes. Transfer the meat to a serving bowl. Place the taco shells on a plate and cover them with a napkin or clean dish towel to keep them warm, setting them out with the bowls of toppings. Let everyone serve him or herself.