Preheat the oven to 180C/350F/Gas 4.
Heat the oil in a heavy-based pan over a medium heat. Add the onion, garlic and rosemary or thyme and fry for 3-4 minutes, or until coloured.
Add the carrots, swede and parsnip and continue to fry for 1-2 minutes, or until just browned.
Add the chopped tomatoes and stock, stir well and bring the mixture to a simmer.
Add the chopped lamb and continue to simmer for 20-30 minutes, or until the volume of liquid has reduced by two-thirds and the sauce has thickened.
Stir in the peas and cook until warmed through.
Transfer the mixture to an ovenproof casserole dish and set aside to cool slightly.
Meanwhile, boil the potatoes in a pan of salted water for 15-20 minutes, or until tender.
Drain well, then mash with a potato masher or ricer.
Add the butter and milk and mash again until smooth and well combined.
Spread the mashed potatoes over the filling mixture in an even layer. Transfer the casserole to the oven and cook for 40-50 minutes, or until the potatoes are golden-brown and the filling is bubbling.
