Begin by bringing a large pot of water to a boil. Cook the spaghetti according to the package directions. Remember to reserve 1 cup of the cooking liquid before draining the pasta.
While the pasta is cooking, heat a large skillet over high heat. Brown the ground beef and onions together until the beef is cooked through. Drain any excess fat from the pan. Add the minced garlic and cook for an additional minute until fragrant.
Stir in the Rotel tomatoes and taco seasoning, ensuring everything is well combined. Bring the mixture to a simmer and let it cook for 3-5 minutes.
Once the spaghetti is cooked, remove the beef mixture from the heat and stir in the sour cream and shredded cheddar cheese until melted and combined.
Add the cooked spaghetti to the skillet along with enough reserved pasta water to achieve a creamy consistency. Stir everything together until well coated and creamy.
Serve the taco spaghetti immediately, garnished with additional cheese and chopped cilantro if desired.