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 34 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. Photograph by Todd Coleman/The Mom 100 Cookbook