Prep: Preheat the oven to 200°C/180°C fan/400°F/Gas 6.
Make the mash. Peel and chop 800 g of Sweet potato into evenly sized pieces.Boil for around 15 minutes, or until tender. Drain well and mash with 80 ml of Whole milk, 1 ½ tablespoon of Butter, ¼ teaspoon of Fine sea salt and ½ teaspoon of Ground black pepper.If you're using leftover mash, you can skip this step.
Cook the vegetables. Heat 1 tablespoon of Olive oil in a large pan. Add 1 Onion (diced), 2 Garlic cloves (crushed), 300 g of Leeks (sliced), 2 Carrots (diced) and 100 g of Mushrooms.Cook for around 8–10 minutes, stirring occasionally, until the carrots have started to soften and the moisture from the mushrooms has reduced.
Make the creamy sauce. Add 250 ml of Vegetable stock (low salt), 1 teaspoon of Dijon mustard, ½ teaspoon of Dried oregano and ½ teaspoon of Marjoram. Simmer until the liquid has reduced slightly.Lower the heat and stir in 200 g of Reduced fat crème fraîche and 25 g of Mature cheddar. The filling should be creamy rather than watery.
Add the leftovers. Stir in 500 g of Leftover turkey, 120 g of Ham and 200 g of Sweetcorn. Mix until everything is coated in the sauce, without boiling the cooked turkey.
Assemble the pie. Transfer the turkey mixture to a large ovenproof dish.
Top the pie: Spoon the sweet potato mash over the filling and spread it out evenly.
Bake: Place in the oven for around 30 minutes, until the filling is bubbling and piping hot throughout.
Add the cheese. Sprinkle over the remaining 25 g of Mature cheddar.
Brown the top: Place under the grill for a few minutes, until the cheese has melted and is golden.
Allow to stand: Leave the pie to stand for around 5 minutes before serving.
Notes
Don't overcook the turkey. It's already cooked, so add it towards the end of preparing the filling. It only needs to be heated thoroughly during baking.
Make sure the filling isn't too watery. Mushrooms release quite a lot of moisture, so cook this off before adding the stock. If the sauce is still thin, let it reduce further before assembling the pie.
Cook raw carrots until nearly tender. They take longer than mushrooms and leeks, so don't rely on the baking time alone to soften large pieces.
Drain the sweet potatoes well. Excess water will make the mash loose and more likely to sink into the filling.
Use leftover gravy. Swap the creamy sauce for leftover turkey gravy if you prefer a richer, more traditional flavour.
Try a pastry topping. Puff pastry, filo or shortcrust are all alternatives if you don't want mashed potato.
Use leftover chicken instead. This makes a delicious chicken and vegetable pie any time of year, not just after Christmas.
Taste before adding salt. Ham, cheese, mustard and stock can all be salty, particularly when you're using Christmas leftovers.