Preheat the oven to 240C/fan 220C/gas 7. Cut a thin slice off each end of the potatoes, to make each end flat. Then cut each potato crosswise into 4 x 3cm slices.   Place the potato slices in a medium bowl, cover with cold water and let them stand for 5 minutes.   Meanwhile, heat a 25cm cast iron or heavy ovenproof skillet over a medium heat for 5 minutes.   Drain the potatoes and pat them dry well with kitchen roll.   Sprinkle the potatoes with the salt and freshly ground black pepper.

Add the oil and 30g of the butter to the preheated casserole or skillet. Swirl the pan to coat the bottom.   Add the potato slices in a single layer and cook for 7 to 10 minutes until the bottoms are golden brown.  Turn the potatoes and cook for another 7 to 10 minutes, until the bottoms are golden brown.

Add the remaining 60g of butter around the potatoes. Then add the thyme, rosemary and garlic.   Carefully pour 118ml of the stock over the potatoes.

Place the pan in the oven and roast, uncovered, for 15 minutes. Remove the pan from the oven.   Carefully tilt the pan and spoon the roasting liquid over the potatoes.   Turn the potatoes and add the remaining stock.

Return the pan to the oven and roast, uncovered, for about 10 minutes until the potatoes are very tender when pierced with a knife.

Transfer the potatoes to a serving platter. Discard the herb sprigs and spoon the roasting liquid over the potatoes.   Sprinkle with additional salt and fresh thyme and/or rosemary (optional).