Preheat oven to 425°F. Line large rimmed baking sheet with foil and lightly spray with nonstick oil.
Using kitchen shears, cut the chicken horizontally to make 6 thin fillets; set aside.
In a shallow dish or pie plate, mix together the melted butter, seasoned salt, dry mustard and Worcestershire sauce. In a separate shallow dish, combine bread crumbs, grated Parmesan cheese, parsley and onion rings.
Dip chicken in the butter mixture, coating both sides. Then dip into the crumbs mixture, coating both sides. Place on prepared baking sheet.
Bake for 20 minutes then carefully flip the chicken and bake an additional 10 minutes or until coating is crisp and chicken is cooked through to 165°F.
While chicken is baking, prepare the spaghetti and heat the sauce according to packaged directions.
Serve chicken with cooked spaghetti and sauce.
For optional mozzarella cheese topping: Before serving, spoon marinara sauce over each chicken breast, then sprinkle a couple tablespoons of shredded mozzarella. Return to oven for 5 minutes to melt the cheese, or broil for 1 or 2 minutes. (We omit this for less calories.)
Nutrition information is an estimate only based on the chicken breast only.