Heat oil in a large casserole pan over high heat. Cook lamb, turning, for 5 mins or until browned. Transfer to a plate.
Add the onion, carrot, celery and garlic to the pan. Cook, stirring, for 5 mins or until the onion softens. Return lamb to pan. Add wine and cook for 2 mins or until reduced by half. Add stock (increase by ¼ cup/60ml if omitting wine), thyme and rosemary. Bring to the boil. Reduce heat to low. Cook, covered, turning occasionally, for 2 hours or until lamb is falling off the bone. Remove from heat.
Meanwhile, to make the shortcrust pastry, place the flour and butter in a food processor and process until mixture resembles fine breadcrumbs. Add the egg yolks and water and process until the dough just comes together. Turn onto a lightly floured surface and shape into a disc. Cover with plastic wrap and place in the fridge for 30 mins to rest.
Preheat oven to 200°C. Roll out the pastry on a lightly floured surface to a 5mm-thick disc. Line a 20cm round springform pan with the pastry. Place in the fridge for 15 mins to rest.
Line the pastry case with baking paper and fill with pastry weights or rice. Bake for 10 mins. Remove the paper and weights. Bake for a further 10 mins or until light golden.
While the pastry is baking, cook the potato in a large saucepan of boiling water for 15 mins or until tender. Drain well. Return to pan. Add half the butter. Use a potato masher to mash until smooth. Stir in the cream. Season.
Transfer the lamb and vegetables to a heatproof bowl. Reserve 1½ cups (375ml) of the cooking liquid. Remove and discard the bones. Coarsely shred the lamb. Melt remaining butter in a medium saucepan over medium heat. Add flour and cook, stirring, for 1 min or until mixture bubbles and becomes grainy. Add the reserved cooking liquid. Cook, stirring constantly, for 3-4 mins or until sauce boils and thickens. Stir in the Worcestershire sauce and tomato paste. Pour over the lamb mixture and stir to combine. Add the peas. Season.
Spoon the lamb mixture into the pastry case. Spoon the mash over the top. Sprinkle with thyme sprigs. Bake for 20 mins or until golden brown and heated through. Cut into wedges and serve with salad leaves.