Preheat your oven to 200°C, 400°F, Gas Mark 6
Fry your onion gently in the olive oil until just softened
Boil the sweet potato in water for about 6-8 minutes until just fork tender
Add the mushrooms and chestnuts to your onions and continue to fry for about 5 minutes until the mushrooms are browned and have released their juices
Add the garlic and sweet potato along with the thyme and chilli flakes to your mushroom mixture, stir well and cook for another minute
Turn off the heat and stir your stilton into the mixture
Leave to cool a little while you prepare your pastry
Lightly grease your dish and roll your pastry out until it is slightly larger than you need. Put your dish on the pastry and press down lightly to leave an imprint. Cut around the resulting imprint with a sharp knife.
Use a star or holly shaped cutter to cut out enough shapes from the pastry to make your pie lid look nice and festive! Place your filling into your dish, top it with your pastry lid and brush with a little milk.
Pop your stars on top, brush with a little more milk and put into the oven for 25-30 minutes until browned and puffy.