Preheat oven to 375 degrees
In a 9 x 13 casserole dish, combine roast, onion, and vegetables.
In a small bowl, mix gravy, diced tomatoes, and Worcestershire sauce.
Pour gravy mixture over the meat and vegetables.
Spread mashed potatoes over the meat and vegetables.
Sprinkle parsley or cheddar cheese over the top of the mashed potatoes.
Bake at 375 for 25 - 30 minutes or until the top begins to brown.