Boil salted water in a large pot. Add spaghetti noodles and cook according to package instructions, about 8-10 minutes. Reserve ½ cup pasta water, then drain spaghetti.

Heat oil in a large skillet over medium-high heat. Brown the lean ground beef with chopped yellow onion for about 5 minutes until meat is fully cooked and onions are translucent.

Stir in taco seasoning and Rotel tomatoes, then pour in reserved pasta water. Simmer for about 5 minutes, stirring occasionally.

Add drained spaghetti to the skillet and toss with the beef and sauce mixture using tongs.

Sprinkle in shredded cheddar cheese and stir until melted and creamy. Cook for an additional minute.

Serve Taco Spaghetti hot, garnished with fresh cilantro and optional toppings like sour cream or salsa.