Peel and slice the potatoes approx 2-3mm thick then place in a large bowl of cold water.
Grease the bottom and sides of a 10 inch nonstick ovenproof skillet with 1 tablespoon of the butter.
In a small saucepan heat the remaining butter, thyme and garlic over a medium heat for a couple of minutes.
Drain the potato slices and pat dry using kitchen paper. Starting in the centre of the skillet arrange ¼ of the potato slices in a circular pattern so they cover the bottom of the pan. Brush with ¼ of butter mixture, season, the sprinkle with ⅓ of the parmesan. Repeat twice more, ending with the cheese, then arrange the final ¼ of potatoes, brush over the remaining butter then season.
Cook over a medium high heat for 5 minutes, then place in the oven to cook for a further 20-25 minutes. Remove from the oven and run a small spatula around the edge of the potatoes, gently shake the pan to loosen the potatoes from the bottom. Leave for 5 minutes to sit at room temperature before carefully inverting onto a serving plate.