Layers of sweet and regular potatoes, buttery onions and fragrant thyme. There’s no need to peel the potatoes (the goodness is in the skin) unless they are particularly gnarly. A well-sharpened chef’s knife is all you need to slice them thinly, or use a mandoline.
70g unsalted butter, plus extra for greasing
2 onions, halved and finely sliced
5 thyme sprigs
1kg potatoes (eg maris piper or king edward), scrubbed clean and sliced as thinly as possible
500g sweet potatoes, scrubbed clean and sliced as thinly as possible
200ml chicken stock, heated
- Preheat the oven to 200C/180C fan/gas 6 and grease a large ovenproof dish (about 1.75-2 litres in volume). Heat 30g butter in your largest sauté pan or saucepan over a medium heat. Add the onions, the leaves from 2 thyme sprigs and a good pinch of salt, then sweat for 5 minutes until softened.
- Add the potatoes, another pinch of salt and cook, stirring gently, for another 4-5 minutes.
- Arrange half the potatoes and onions in the baking dish, season and dot with 20g butter. Arrange the remaining potatoes and onions on top (it’s worth taking a bit of time to make the very top layer look pretty). Pour over the hot stock, then season and dot with the remaining 20g butter and scatter with the remaining thyme sprigs.
- Bake the potatoes for 50-60 minutes, until golden and bubbling. Stand for a few minutes before serving.
TIP If cooking the potatoes in the same oven as the lamb, place them on a lower shelf for the last 30 minutes of the lamb cooking. While the lamb rests, place them on the upper shelf and turn the heat up to 200C/180C fan/ gas 6, cooking for a final 30 minutes.