Add beef and onion to a large skillet and place it over medium high heat. Cook for 4-5 minutes, until the meat is no longer pink and the onions have softened. Add the taco seasoning and mix until combined.
Remove the meat mix off the heat. Place a heaping amount onto each taco. Add shredded cheese but do not close the tacos.
Clean the pan, add oil, and place it over medium heat. Once hot, place the prepared tortilla flat into the pan. Cook for 20-30 seconds, or until the tortillas are pliable. Carefully fold them in half.
Cook the tortillas for 1-2 minutes per side, until crispy and golden. Repeat the process until all the tacos are cooked.
Carefully lift one side of the taco up and fill with tomatoes, avocado, and sour cream.
* I like using a Mexican cheese blend, but any cheese that melts well works.