Step 1
Preheat oven to 150°C. Heat half the oil in a large ovenproof casserole pan over high heat. Add half the lamb and cook, turning, for 5 mins or until golden brown. Transfer to a heatproof bowl. Repeat with remaining lamb.
Step 2
Heat half the remaining oil in the pan. Add half the onion, celery and carrot and cook, stirring, for 5 mins or until vegetables are just tender. Add the garlic and tomato paste and cook, stirring, for 1 min or until aromatic. Return the lamb to the pan with the rosemary, thyme, chicken stock and tomatoes. Bring to the boil. Season. Remove from heat.
Step 3
Bake, covered, for 1½ - 2 hours or until lamb is very tender. Set aside to cool slightly. Use 2 forks to coarsely shred the lamb.
Step 4
Meanwhile, place the potato in a large saucepan and cover with cold water. Place over high heat and bring to the boil. Cook for 15 mins or until potato is tender. Drain well. Set aside for 2-3 mins to steam. Return to the pan with the butter. Use a potato masher or fork to mash until smooth. Add the milk and stir to combine. Season well. Cover to keep warm.
Step 5
Heat the remaining oil in a medium frying pan over medium-high heat. Add the remaining onion, celery and carrot and cook, stirring for 5 mins or until tender. Add to the lamb mixture with the peas and stir to combine. Season.
Step 6
Increase oven to 180°C. Spoon mashed potato over the lamb mixture. Bake for 30 mins or until the top is golden brown and mixture is heated through. Serve with mixed salad leaves.