This comforting pie makes a great vegetarian centerpiece, ideal for a celebration. It benefits from being assembled the day before, receiving a good amount of time to set in the fridge and thus involving a lot less work on the day you plan to enjoy it. You can also bake the whole thing a few hours in advance, then just reheat it in the oven for 20 minutes, if you like. Use any extra pastry trim to cut out fun, festive shapes like holly leaves or stars to personalize your pie. Serve this alongside roasted veggies or a big leafy salad.