Heat the oil in a frying pan over a medium heat and fry the lamb for 4-5 minutes, or until golden-brown. Season, to taste, with salt and freshly ground black pepper.
Add the onion and continue to fry for a further 2-3 minutes. Pour in the wine, bring to the boil, and cook for 1-2 minutes, or until the volume of the liquid has reduced by one-third. Add the stock, return the mixture to the boil, then reduce the heat until the mixture is simmering and simmer for 2-3 minutes.
Pour the mixture into a small ovenproof dish and set aside to cool.
Meanwhile, cook the swede in boiling water for 5-8 minutes, or until tender, then drain well. Mash the swede until smooth and season, to taste, with salt and freshly ground black pepper. Spoon the swede mash on top of the lamb mixture.
Dot the swede mash with the butter. Transfer the dish to the oven and bake for 8-10 minutes, or until golden-brown and crisp on top.