Unwrap the lamb and pat dry with kitchen paper. Open the joint out and slice it into 2 large pieces. Warm a large frying pan (or casserole dish if you’re cooking in the oven) for 2 mins, then add 1 tbsp olive oil and the lamb. Fry for 12-15 mins, turning every so often, till the lamb is browned all over.

While the lamb fries, trim the roots and top 3cm off the top of the leek, then halve it and rinse out any grit. Finely slice the leek. Peel and finely chop the onion. Trim and peel the carrots and slice them into 1cm-thick rounds. Trim the dry ends from the celery sticks and slice them. From this point on, follow the method for the slow cooker or the hob.

In the slow cooker^ Take the lamb off the heat and set aside, still in the frying pan. Fill your kettle and boil it. Thickly slice the mushrooms. Trim and coarsely grate the courgette. Finely chop the parsley stalks, setting the leaves aside for later.

Add half the veg and parsley stalks to a 3½ ltr slow cooker. Tip in the pearl barley, season with a pinch of salt and pepper and stir mix. Pop the browned lamb on top of the veg. Top with the remaining veg and parsley stalks. Pour in 500ml hot water from the kettle. Pop a lid on the slow cooker, set to medium and cook for 5-6 hrs. For a faster stew, set the slow cooker to high and cook for 4-5 hrs, or set to low and cook for 7-9 hrs.

When the casserole has cooked for 4 hrs, sift the flour into a bowl. Add 1 tsp baking powder and a pinch of salt. Whisk to mix. Chop the butter and rub it into the flour with your fingertips to make a fine, sandy mixture. Stir ½-1 tbsp horseradish mustard into the flour (it’s spicy, so use as much or as little as you prefer), then add 6-8 tbsp cold water and stir together with a fork to make a soft dough. Roll into 8 round dumplings.

Taste the stew and add a pinch more salt or pepper if needed. Stir to break up the lamb into shreds. Carefully rest the dumplings in the slow cooker. Put the lid back on and cook for another 1 hr. Ladle the stew and dumplings into warm bowls to serve, garnished with parsley leaves.

In the oven^ Heat your oven to 160°C/Fan 140°C/Gas 3. Pop the browned lamb onto a plate. Add the leek and onion to the casserole dish with 2 tbsp water. Fry, stirring now and then, for 5-8 mins till the veg have started to soften. Thickly slice the mushrooms.

Stir the carrots, celery and mushrooms into the leek and onion and season with a pinch of salt and pepper. Fry, stirring every so often, for 5 mins. If they start to stick, add a splash of water.

While the veg fry, trim and coarsely grate the courgette. Finely chop the parsley stalks, setting the leaves aside for later. Fill your kettle and boil it.

Add the courgette and parsley stalks to the dish. Tip in the pearl barley. Return the lamb to the dish with any juices from the plate. Add 1½ ltr boiling water, pop a lid on the dish and slide into the oven to cook for 2 hrs. No casserole dish? Ladle the stew into a large ovenproof dish and cover with foil.

When the casserole has cooked for 1 hr 45 mins, sift the flour into a bowl. Add 1 tsp baking powder and a pinch of salt. Whisk together with a fork or whisk to mix. Chop the butter and rub it into the flour with your fingertips to make a fine, sandy mixture. Stir ½-1 tbsp horseradish mustard into the flour (it’s spicy, so use as much or as little as you prefer), then add 6-8 tbsp cold water and stir together with a fork to make a soft dough. Roll into 8 round dumplings.

Carefully rest the dumplings in the dish, on top of the stew. Slide back into the oven, without the lid, and bake for 30 mins. Ladle the lamb stew and dumplings into warm bowls, and serve garnished with the parsley leaves.