Heat oven to 170C/150C fan/gas 3½. Butter a medium-sized gratin dish (a round or oval one about 22cm wide is ideal) with a little of the butter. Slice the potatoes into very thin rounds – a mandolin or the sharp slicing blade of a box grater is good for this. As you slice, stack the potatoes in piles and pack them into the dish starting from the outside, building a trail around the dish and then into the middle. You want all the potatoes standing on their edges like dominoes. Slip the bay leaves between some of the slices.
Drizzle the cream and the rest of the butter over the potatoes, then sprinkle the thyme over the top. Season well and cook for 1 hr until the potatoes are tender and cooked through, then turn the oven up to 200C/180C fan/gas 6 for 10-15 mins to crisp up the top of the potatoes.