In a large skillet, heat the oil over medium heat, and add the onion. Season with 1/8 tsp salt, and cook for 5 minutes until slightly softened.
Add the garlic and cook for 30 seconds, until fragrant.
Turn the heat up to medium high and add the ground beef. Season with 1/2 tsp salt and 1/4 tsp black pepper, and brown the beef for 3-4 minutes, stirring frequently.
Add the taco seasoning, and keep cooking for 2-3 minutes, until no pink remains in the beef.
Add the cornstarch water slurry and the tomato paste, and cook for 3 more minutes, until the cornstarch thickens the liquid in the pan. Taste the beef, and make any necessary adjustments with salt and pepper before removing the skillet from the heat.
Serve the ground beef in taco shells with the toppings of your choice. Enjoy!