Fill a large pot with water and fill with pumpkin and sweet potato. Bring to the boil and cook until tender. Drain. Then mash with a pinch of salt and pepper to taste. Set aside.
Preheat oven to 180 degrees Celsius (fan-forced).
Place all leftover roast beef and vegetables into a large saucepan on medium heat. Saute for 3-4 minutes then add gravy, stock, Worcestershire and tomato paste. Simmer until sauce thickens (5-10 minutes). This is your pie filling.
Place pie filling into oven safe casserole dish. Top with mash and carefully spread out evenly. Top with cheese and bake for 12-15 minutes until cheese is melted and browning.
Remove from oven and sit for 5 minutes before serving.