Heat oven to 190C/170C fan/gas 5. Tip the cream, milk and garlic into a large saucepan and bring to a simmer. Slice the 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 the 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.