Preheat the oven to gas 6, 200°C, fan 180°C. Rub the butter over the inside of a large baking dish (20x30cm/8x12in). Layer the potatoes and pumpkin or squash in the dish, scattering the sage and plenty of seasoning between each layer. In a small pan, bring the milk and cream to a simmer then pour over the layered vegetables. Cover with foil and bake for 40 minutes.
Remove the foil and scatter over the remaining sage. Bake for a further 15 minutes or until the potato is completely tender and the top is slightly browned. This will keep warm covered in foil while you cook the beef.
Butternut squash gives a firmer texture to this dish as it releases less water than pumpkin as it cooks, but both give a sweet, nutty flavour to the bake.
Freezing and defrosting instructions
Can be kept in the freezer for up to 3 months. Always defrost and heat thoroughly before eating. Never re-freeze food that has already been frozen; this includes raw and cooked ingredients such as meat, fish and poultry.