Weigh the Welsh Lamb joint and calculate the cooking time. Place the Welsh Lamb on a board and make several slits into the joint.
Line a large roasting tin with foil. Place the oil, onions, seasoning, mint, vinegar and brown sugar into the roasting pan, add the Welsh Lamb and roll around to coat thoroughly. Roughly squash some of the onions, mint and sugar into some of the slits – leave to marinate for 2 hours, covered and in the fridge.
Remove from fridge, spoon over any marinade juices, scrunch the foil very loosely around the bottom of the joint and cook for the calculated cooking time. Baste the Welsh Lamb joint a couple of times during cooking. Mix all the ingredients together for the mint sauce and allow to stand at room temperature for about 20 minutes to infuse flavours.
Remove joint from oven and allow to stand for about 15-20 minutes before carving. Serve on a bed of steamed spring vegetables, and spoon over the cooking juices.