Beef FillingMix together all the ingredients except the meat. Stir the spice mix well to make sure that all spices have been blended well.Crumble the ground beef into a large skillet (preferably nonstick) and brown, stirring well. Remove from the heat. Dump the meat into a strainer in the sink, rinse with hot water, and drain off the water and grease from the beef. Return the beef to the pan. Stir in the spice mix and 3/4 to 1 cup water. Simmer on medium-low heat for 20 minutes, until most of the moisture has cooked away. Remove from heat before the meat is completely dry.TacosIf you like, you can refresh the taco shells in the oven at 350 degrees F for 7 to 10 minutes.To assemble the tacos, into each shell spoon a couple of tablespoons of the meat. Top with lettuce, cheese, tomatoes, and sour cream if desired. Serve immediately.