Cook the Beef: In a large skillet, add garlic, onions, and ground beef. Cook until the beef is browned, breaking it up as you cook. Drain out excess fat.
Add Taco Seasoning and Liquids: Sprinkle the taco seasoning over the beef, then add the diced tomatoes (with liquid), water, and uncooked spaghetti.
Cook the Pasta: Bring the mixture to a boil. Push the spaghetti down so it's submerged in the liquid. Reduce the heat to a simmer, cover the skillet, and cook for about 15 minutes, stirring occasionally, until the spaghetti is cooked and most of the liquid is absorbed.
Finish Cooking: Once the spaghetti is done, remove the lid and cook for an additional minute to reduce any remaining liquid.
Add Cheese: Sprinkle the shredded cheddar cheese over the pasta, cover with the lid, and let it sit for 2-5 minutes until the cheese melts.
Serve: Garnish with chopped parsley and serve immediately.