Melt the butter in a saucepan over medium heat. Whisk in the flour and cook for 2 minutes.
Slowly whisk in the broth and cooking liquid. Add Worcestershire, thyme and pepper and simmer for 5 minutes until lightly thickened.
Add the cooked pot roast to the gravy in the saucepan. Cover and reheat over medium-low heat, turning gently, until the beef reaches 165°F throughout. Allow approximately 10–15 minutes for refrigerated, sliced or shredded beef; the thermometer reading determines when it is ready.
Serve immediately, or transfer the hot beef and gravy to a preheated slow cooker set to warm for holding at 140°F or above.
Meanwhile, boil the frozen noodles according to their package directions and drain them well.
Divide the noodles among bowls and ladle the hot beef and gravy over them. Add parsley only if using and serve immediately.