Pre-heat the oven to 200° C.
Melt most of the butter in a large saucepan or casserole dish over a medium heat. Sweat the sliced leeks for 6-7 minutes until softened, then add the beef and increase the heat, browning it well all over. Add the stock, bring the mixture to the boil, then reduce to a simmer for 15 minutes.
Adjust the seasoning to taste then remove from the heat. Spoon into a 12 inch x 6 inch ceramic baking dish. Layer the potatoes on top, seasoning well in between the layers.
Dot the top of the potatoes with the remaining butter, then cover the dish with aluminium foil and bake for 40-45 minutes until the potatoes are soft. Remove from the oven once they are soft and discard the foil.
Sprinkle the grated cheeses and dried oregano on top. Return the dish uncovered to the oven for 15 minutes until the cheese is golden and melted. Remove from the oven and leave to stand for a few minutes before serving.
Freezing and defrosting guidelines
Once the dish has cooled completely, transfer it to an airtight, freezer-safe container, seal and freeze for up to 1-3 months. To serve, defrost thoroughly in the fridge overnight before reheating. Loosely cover with foil and bake until dish is thoroughly heated through. Reheat until piping hot.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.