1. For the filling, place a large skillet over medium-high heat. Add a generous glug of oil, the ground beef and a pinch of salt. Cook undisturbed for about a minute or two, then break up the meat into a smaller mince. Continue cooking, stirring occasionally, until the meat just starts to brown, about 6 minutes. 
2. Increase the heat to high, then add the sliced onions. Cook for about 6 minutes until meat and onions begin to brown. Add the garlic and cook for another 2 minutes. Add about ½ cup of water to the beef, scraping up any brown bits from the bottom of the pan. Add about ¾ of the jar of salsa and stir through to coat (reserve the remaining salsa for serving). Cook, stirring constantly, until the salsa reduces and coats the taco meat, about 4-5 minutes. Remove from heat and set aside.
3. To finish and assemble, heat the shells following the package directions. Fill the warm taco shells with the taco meat, then layer on the toppings to suit your taste and enjoy!