Preheat the oven to 400 degrees.

Place potatoes on a lined baking sheet and toss with olive oil, thyme, salt and pepper. Bake for 20 – 25 minutes or until baked through. 

While the potatoes are in the oven, chop up your leftover pot roast into bite sized chunks and toss in a 9 x 13 baking dish. 

Add corn, carrots and peas. Once the potatoes are done cooking, add those as well. 

Pour 3 cups of warm beef gravy over the beef and vegetables and mix the gravy in with a spatula so everything is coated. 

Roll out the puff pastry dough giving it an extra ½  to fit across your baking dish. 

Place the puff pastry on top. 

Whisk together the egg with the milk and brush a little egg wash on the puff pastry.

Bake until the pastry is nice and golden, about 25 to 30 minutes. 

Sprinkle with fresh herbs and serve.