Heat a large skillet over medium-high heat with the olive oil. Once hot, add the ground sirloin and begin breaking it down with a wooden spoon. Cook until browned and crispy, stirring often, about 10 minutes. Add the onions and garlic and cook until softened, about 5 minutes. Stir in the taco seasoning and tomato sauce. In a small bowl, combine the cornstarch and water until dissolved and then stir it into the beef mixture. Cook over low, until thickened, about 5 minutes.
Warm the taco shells in a preheated 350 degree oven for about 5 minutes before serving.
To assemble the tacos, fill the warmed shells with the beef mixture and top with shredded lettuce and cheese, or use whatever taco toppings you love!