Add pasta to salted boiling water and cook until tender; drain.
In a 12-inch skillet, heat oil to medium heat and saute onion 2-3 minutes until it just starts to turn translucent. Add ground beef and cook until meat is nicely browned; drain. Stir in taco seasoning, cayenne (optional), diced tomatoes, salsa, water, and green pepper or jalapeño, if desired. Bring to a boil; reduce heat and simmer 5 minutes.
Add pasta, 1 cup cheese, and sour cream; stir well to combine. Continue cooking until cheese has melted. Top with remaining cheese and sprinkle with crushed tortilla chips and cilantro. Drizzle with 2-3 tablespoons taco sauce. Serve immediately.