Preheat the oven to gas 5, 190C/fan 170C/gas 5. Lightly grease a large ovenproof dish with the butter. Put the potatoes in a large saucepan with the onion and garlic and pour over the milk (add a little more to cover if needed). Season with salt, bring to the boil, then reduce to a simmer and cook for 10 minutes. Remove from the heat.

Using a slotted spoon, transfer enough potatoes and onions to make an even layer in the dish, leaving the milk in the saucepan. Add a layer of the gammon and a sprinkle of cheese. Repeat with the remaining ingredients, building more layers, finishing with the potatoes and a sprinkling of cheese.

Stir the cream and nutmeg into the infused milk mixture. Check the seasoning and add, to taste, if required.  Pour the mixture over the potatoes. Bake for 30 minutes until golden and bubbling.