Slice the potatoes to about 3mm thick (you can use mandoline ). Set them aside in a bowl with cold water to ensure they don't brown. In a saucepan heat the cream, milk, garlic, thyme, salt, and pepper. Bring to the boil, then reduce heat to a simmer for a few minutes, put it to a sieve and discard seasoning. Lay a square oven dish with baking paper. Fill it with potato slices making sure you overlap. Make 6-7 layer to make 5cm hight. Pour over milk and cream mixture and ensure the top layer is cover with cream. Cover with baking paper. Place your dish on the middle tray in a preheated 160 C oven for at least 1 hour or until pale golden on the top. With another tray press down the top of the potatoes together with backing paper to make them dense.Let them cool down and then in the fridge overnight. Next day turn the potatoes upside down over a chopping board. Remove the baking paper on top. Cut potatoes with a thin knife in cubes about 5x5cm. Place them in a shallow tray and brown in a preheated oven before service