Preheat oven to 180C. In a large bowl mix together cream, gruyere, garlic, pepper, salt and thyme.
Thinly slice potato (easiest done on a mandolin) and place in the cream mixture, tossing well to coat each potato slice.
Butter a 20cm cast iron pan. Arrange the potatoes hasselback style in the pan, tightly packed.
Cover the pan with foil, place on a tray (to catch any drips) and bake for 1 hour.
Check the potatoes for doneness with a skewer (they should be tender). They may take an extra 15 minutes.
Remove foil and bake for a further 30 minutes to brown and crisp up.
Serve bake sprinkled with grated parmesan.
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