Start by bringing a large pot of salted water to a boil. Add the spaghetti and cook it according to the package instructions until al dente. Before draining, scoop out about ½ cup of the pasta water and set it aside—this little step helps bring the sauce together later.
While the pasta is cooking, heat a large skillet over medium-high heat. Add the ground beef and chopped onion, and cook until the beef is browned and the onion is soft, about 5 minutes. Stir in your taco seasoning, the can of tomatoes (with their juices), and that reserved pasta water. Let everything simmer for about 5 minutes so the flavors meld.
Once the pasta’s ready, toss it right into the skillet. Add the shredded cheese and give everything a good stir until the cheese melts into a creamy, taco-infused sauce. Serve it hot, topped with fresh cilantro and anything else you love—diced tomatoes, a dollop of sour cream, or even a little extra cheese on top.