Heat oven to 160C/fan 140C/gas 4. Line an 20cm (8 inch) square tin with greaseproof paper, making sure there are no holes for the liquid to seep through, then butter the paper.
Peel and slice the potatoes to the width of a £1 coin and pat dry.
Pour the milk and cream into a pan, then add the garlic and thyme.
Heat to boiling point, cool a little and strain into a jug. Sprinkle with nutmeg and keep warm. You can prepare this in advance and reheat.
Layer half the potatoes in the tin, overlapping the slices, sprinkling each layer with a little salt and pepper.
Pour over half the liquid and finish layering, then add the rest of the liquid and scatter over the cheese.
Bake for 1-1¼ hrs until the potatoes are tender and the top is golden. Leave to stand for 5 mins, then cut into 6 portions and serve.