Preheat the oven to 400°F and lightly grease an 8x8 baking dish.

Slice the potatoes thinly, using a mandolin or sharp knife. Place the slices in a colander and rinse, then set aside to drain.

Heat a large pan over medium heat, melt the butter. Add the garlic and cook for just 30 seconds, until aromatic.

Whisk in the flour and let it cook for 1-2 minutes. Stir in the heavy cream and whisk rapidly until thickened, to avoid clumps forming.

Stir in the Parmesan, reserving a few tablespoons for the end, thyme, and salt and pepper.

Remove from the heat and let cool slightly.

Dry off the potatoes, if necessary, then add them into the pan with the sauce. Toss to coat.

Arrange the potato slices upright in the baking dish. Don’t pack them too tightly, leave some space for them to fan out a little. Pour on any remaining sauce. Sprinkle a the rest of the Parmesan.

Cover with aluminum foil and bake for 30 minutes. Then remove the foil and bake uncovered for another 30 minutes or until the potatoes are golden brown and cooked through.

Remove from the oven and let cool for about 10 minutes before serving.