1 . Stage
Heat the oil in a frying pan over a low heat. Add the onion, cover, and fry for 15 mins, stirring occasionally. Cooking the onions over a low heat keeps them moist and brings out their natural sweetness. You could swap in a finely sliced leek if you prefer. To give it an extra layer of flavour, add a little finely chopped rosemary or thyme for the final min of cooking.
2 . Stage
Using a mandolin or sharp knife, carefully cut the potatoes into very fi ne 3mm slices. Preheat the oven to gas 5, 190°C, fan 170°C. Grease a baking dish with the butter. Cut the potatoes into as uniform slices as possible, so that they cook evenly. You want them to be as thin as possible too, so they don’t take a long time to cook through and so that you have lots of layers for the cream to coat and flavour
3 . Stage
Pour the milk and cream into a large saucepan, add the garlic cloves and bay leaf. Bring to the boil, then remove from the heat and add the nutmeg and some seasoning. Remove the garlic and bay leaf. Infusing the milk helps flavour the whole dish, making it beautifully aromatic. Using half milk and half cream helps to balance the richness, but you can alter the ratio if you prefer.
4 . Stage
Add the sliced potatoes and onion to the milk, stir to coat. Transfer to the dish with a slotted spoon, layering as you go. Use a fish slice or slotted spoon to transfer the potatoes and onion to the dish, leaving the infused milk and cream in the saucepan.
5 . Stage
Pour over the infused milk and scatter over the cheese. The cream mixture should come to just below the potato. Swap the cheese if you like: Roquefort will add a punchy flavour, while mozzarella will add gooeyness.
6 . Stage
Bake for 1-1 hr 15 mins, covering with foil once golden. It’s ready when the top is golden and bubbling and the potatoes are tender all the way through. Test if it’s ready by piercing the centre with a sharp knife, it should go through easily. If not, bake for 10 mins more, then check again. Leave to stand for at least 5 mins, then top with the thyme sprigs, if you like.