In a large skillet, cook beef and onion over medium heat until meat is no longer pink; drain. Stir the pasta, contents of seasoning packet, salsa, water, chili powder, salt and cayenne into the skillet.
Bring to a boil. Reduce heat; cover and simmer for 15 minutes or until pasta is tender, adding more water if necessary. Stir in tomatoes; sprinkle with cheese. Cover and simmer for 3-4 minutes or until heated through. Serve with sour cream if desired.
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