Heat olive oil in a large 12-inch skillet over medium heat. Add ground beef and chopped onion. Cook, breaking meat into crumbles, until meat is browned and onions are soft—about 6–8 minutes. Drain any excess grease if necessary.

Sprinkle in taco seasoning, then stir in the entire can of Rotel tomatoes, juice and all. This adds the dish’s signature spicy, tangy base.

Break the spaghetti noodles in half and scatter them into the pan. Pour in 3 cups of water, stirring so that most noodles are submerged.

Bring mixture to a gentle boil over medium-high heat. Once boiling, reduce heat, cover, and simmer for 15 minutes, stirring halfway through to prevent sticking and ensure even cooking.

Remove the lid, turn off the heat, and stir in half of the cheddar to melt into every noodle. Sprinkle with the remaining cheese and fresh cilantro. Let it rest for a few minutes before serving.