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 beef, and cook, stirring and making sure to really break it up into small crumbles, until it is browned throughout, about 5 minutes. Transfer the beef to a colander and drain off any liquid.

Return the beef to the pan over medium high heat and add the spice mixture. Stir for a minute or two until 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 saucy 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 dish towel to keep them warm, and set out with the bowls of toppings. Let everyone serve themselves.