Pat the lamb shanks dry and rub all over with salt and pepper.
Heat olive oil in a large frying pan over a medium-high heat and sear the shanks until browned on all sides. Transfer to the slow cooker.
Add the onions, celery, potatoes, red wine, stock, and rosemary to the slow cooker.
Cover and cook on low for 8 hours or high for 4–5 hours, until the lamb is tender and falls apart easily.
Remove the lamb shanks from the slow cooker and shred the meat, discarding any bones or sinew. Set the bones aside if using them for presentation later.
Stir the shredded lamb back into the slow cooker. If the mixture is too thin, transfer the liquid to a saucepan and simmer until thickened to your liking. Alternatively, whisk 1 tablespoon of cornflour with 1 tablespoon of water and stir through to help thicken.
Preheat the oven according to your puff pastry packet instructions.
Spoon the lamb mixture into two individual pie dishes. Top each with a sheet of puff pastry, trimming or folding the edges neatly to fit.
(Optional) If using bones for presentation, cut a small cross in the centre of each pastry lid and insert the clean bone before baking.
Press the edges with a fork to seal.
Bake as per pastry packet instructions, or until the pastry is puffed, golden and crisp.
Allow the pies to rest for 10 minutes before serving.

• Add a handful of frozen peas or chopped carrots for extra texture and colour.

• For a richer flavour, replace half the stock with additional red wine.

• These pies can also be made using leftover slow-cooked lamb — just skip to the assembly step.