PREP: Prepare everything for cooking the beef filling, because the process goes quickly! Dice the onion, mince the garlic, and stir together all of the seasonings. Place taco shells in a 9x13-inch baking pan (all 12 should fit) and set aside. Preheat the oven to 375 degrees F.
BEEF: Heat oil in a large skillet (cast iron if possible) over high heat. Add onion and cook for 2-3 minutes or until light golden. Add in the garlic and cook for 30 seconds.
BEEF CONT.: Add ground beef and cook for 2-4 minutes, breaking it up into small pieces as you go, until meat is no longer pink. Avoid stirring/breaking beef up too much-- allow it to brown too. If there is a lot of extra grease (from higher-fat beef), drain off. Add in all the seasonings: chili powder, paprika, cumin, cayenne pepper, oregano, garlic powder, salt, and pepper. Stir for another 1-2 minutes or until beef is completely cooked through. Add in the tomato sauce and stir until thickened, another 1-2 minutes. Remove from heat and set aside to slightly cool.
STUFF SHELLS: Divide the beef evenly between the 12 shells. Add cheese on top of each ( I add about 2-3 tablespoons of a Monterey jack and extra-sharp Cheddar blend per taco ). Bake for 6-8 minutes or until cheese is melty.
SERVE: While tacos are cooking, set out the toppings in individual bowls or on a large platter. Our favorites: chopped cherry tomatoes, iceberg lettuce, sliced avocado, chopped cilantro, lime wedges, and a scoop of sour cream. Other ideas: guacamole, taco sauce, salsa, pico de gallo, chopped onions, etc. Serve tacos hot out of the oven and let everyone add whatever toppings they like best. Enjoy with a squeeze of fresh lime!