Heat oven to 200C/180C fan/gas 6.
Tip the double cream, garlic cloves and the thyme into a large saucepan and bring to a simmer. Then remove from the heat and leave it to infuse for 10 minutes.
After 10 minutes sieve the cream to remove the garlic and thyme and pour it back in the pan. Season it.
Peel and then slice the potatoes very finely, about 3mm, add them to the cream and simmer for about 10 mins.
Gently stir to separate the potato and stop it sinking and catching on the bottom of the pan.
Place half of the potatoes and cream in an ovenproof dish.
Sprinkle half of the cheese and cover with the rest of the potatoes and cream mixture.
Scatter over the rest of the gruyère cheese then bake for 30 mins until the potatoes are soft and the top is golden brown – increase the heat for 5 mins if not brown enough.