Lamb Boulangère

1 large onion, finely sliced
1.5kg waxy potatoes, peeled and finely sliced (you can do this on a thicker setting of a mandoline if you like)
3 garlic cloves, crushed
80g unsalted butter, cut into small chunks
4 rosemary stems, 3 left whole 
1 picked into small sprigs
400ml good-quality liquid chicken stock
1 lamb shoulder, around 2kg
1. Mix the onion, potatoes (save a few for the top) and garlic together in a bowl and season well with salt. Tip into a large roasting dish, Portmeirion for AGA Roaster with Pouring Lip is perfect for this. Tuck in the butter chunks and whole rosemary stems. Layer the reserved potatoes over the top to cover everything, then pour over the chicken stock.

2. Cut small slits all over the top of the lamb, then tuck in the smaller rosemary sprigs. Season the lamb all over, then gently sit it on top of the potatoes. Transfer to the bottom rack of the baking oven, and cook for 5 hours, then move to the simmering oven for a further hour to allow everything to rest while keeping warm.

3. Once the lamb is completely tender and the potatoes are golden brown and crisp on top, remove the lamb and shred it with two forks. If there’s still a lot of stock in the potatoes (it will vary depending on potato variety), gently pour off a bit, but keep some as it forms a delicious sauce.

4. Serve the potatoes with the lamb and some seasonal greens.