In a 12-inch skillet, cook the ground beef and onion over medium-high heat until no pink remains while breaking up with a spoon. Drain any fat. Add the taco seasoning and cook for 1 minute more.
Add the broth, milk, salsa, and bell pepper.
Bring to a boil, add the spaghetti and cover. Reduce the heat to medium low and let simmer covered, stirring occasionally, for 12-15 minutes or until the spaghetti is tender.
Remove from the heat and let rest uncovered for 5 minutes.
Add the cheese and stir until melted. Taste and season with additional salt and pepper or salsa if desired.
