Heat oven to 190C/170C fan/gas 5.
Tip 500ml double cream, 500ml milk and 3 garlic cloves into a large saucepan and bring to a simmer.
Slice 8 large potatoes very finely, about 3-4mm, add them to the cream and simmer for 3 mins until just cooked.
Gently stir to separate the potato and stop it sinking and catching on the bottom of the pan.
Remove the potatoes with a slotted spoon and place in a wide shallow ovenproof dish so that they are about 5cm in depth.
Pour over the garlic-infused cream (discarding the garlic) – just enough to seep through the layers and leave a little moisture on the surface.
Scatter over 100g grated gruyère cheese, if using, then bake for 30 mins until the potatoes are soft and browned – increase the heat for 5 mins if not brown enough.