Heat oven to 200C/180C fan/gas 6. Slice the peeled potatoes as finely as possible on a mandolin, with the fine slicing blade of a food processor, or with a very sharp knife. (The more finely you slice them, the quicker and more evenly they will cook.)

Pat the potatoes dry with a clean tea towel and tip into a large bowl. Pour over the melted butter, sprinkle over the thyme and some salt (preferably sea salt or rock salt), and toss until each slice is coated in butter.

In a roasting tin, measuring about 20 x 20cm, assemble the potato slices vertically, tightly packed together like a deck of cards, until the dish is full and you’ve used all the potatoes. Sprinkle with a little more salt and place in the oven for 45-50 mins until crisp and cooked. Use a skewer or small sharp knife to check that the potatoes are completely soft and cooked in the centre. Serve from the tin alongside the lamb.