Heat your Dutch oven over medium-high heat. Add the ground beef and cook, breaking it up with a wooden spoon, until no longer pink, about 5 minutes. Add the diced onion and cook until softened, 3-4 minutes. Stir in the garlic and cook for 30 seconds until fragrant. Drain excess fat if needed.

Sprinkle the taco seasoning over the beef and stir to coat evenly. Pour in the beef broth, diced tomatoes with their juices, and tomato sauce. Stir well and bring to a boil.

Add the broken spaghetti, pressing down gently so it is mostly submerged. Reduce heat to medium-low, cover, and simmer for 12-14 minutes, stirring once halfway through, until the pasta is al dente and most of the liquid is absorbed. The sauce will look loose but thickens as it stands.

Remove from heat and stir in 1 cup of the shredded cheese until melted. Sprinkle the remaining 1/2 cup on top, cover, and let stand for 2 minutes to melt. Serve hot with sour cream and cilantro on the side.