Heat the oven to 200°C. Trim any fat off the lamb, cut the meat into 2.5 cm (1 in) cubes and arrange them in a large roasting tin with the onions or shallots and potatoes, cut in half if large.
Squeeze the juice from four of the lemon wedges, whisk it into the oil and drizzle it over the meat and vegetables. Tuck all eight lemon wedges, the garlic and half of the herbs into the dish and season to taste. Cover the tin tightly with foil and cook for 45 minutes.
Remove the foil and the herbs and discard them, then increase the oven heat to 220°C. Add the tomatoes and remaining herbs and baste the meat and vegetables with the mint sauce. Roast for a further 15-20 minutes, uncovered, until the meat and vegetables are browned and tender.
Divide the lamb and vegetables between four plates, spoon on any roasting juices and serve.