Preheat the oven to 180C/350F/Gas 4.
For the lamb, heat the oil in an ovenproof frying pan. Season the lamb well with salt and freshly ground black pepper and fry on both sides until browned, about 3-4 minutes.
Add the rosemary and thyme sprigs, the garlic and sweet potatoes to the pan. Transfer to the oven to continue cooking for about 10-12 minutes, or until the lamb is cooked to your liking and the sweet potato is tender. Remove from the oven and allow to rest before slicing.
For the garlic red wine sauce, melt the butter in a saucepan and gently fry the onion and garlic until translucent. Add the brandy and simmer until reduced.
Pour in the red wine and simmer until reduced by half, then add the thyme and stock and continue to cook for a further 3-4 minutes. Finally stir in the cream.
To serve, pile the roasted sweet potatoes onto a serving plate and place the lamb slices on top. Pour over the garlic red wine sauce and garnish the plate with the cloves of roasted garlic from the lamb dish.