In a large pot or Dutch oven, heat olive oil over medium heat.
Add chopped onion and sauté until softened, about 3 minutes.
Add ground beef and cook until browned, breaking it apart with a spoon.
Stir in taco seasoning, beef broth, and Rotel (with juices).
Break spaghetti in half and stir into the pot.
Bring to a boil, reduce heat, and cover. Simmer for 15 minutes, stirring occasionally, until pasta is cooked and liquid is absorbed.
Remove from heat and stir in shredded cheese until melted.
Garnish with chopped cilantro and tomatoes if desired. Serve warm.