Before you begin cooking, bring the lamb breast up to room temperature.
Preheat the oven to 180 C / Gas 4.
Melt the the butter in a frying pan over a medium heat and add the onion and garlic. Cook and stir for 5 minutes until the onions are soft then add the mushrooms and cook until the liquid evaporates. Transfer to a mixing bowl and allow to cool slightly.
To the mixing bowl add the herbs, breadcrumbs and egg. Season well with salt and pepper then mix together with hands. The stuffing should hold together and not be too dry or wet.
Prepare the lamb breast by seasoning generously on both sides then rubbing all over with olive oil. Lay skin side down on a work surface and spread the stuffing mixture over the surface of the lamb. Carefully roll up the breast and secure with kitchen string. Weigh the stuffed lamb and record. Place the lamb on a roasting rack inside a roasting tin.
Roast in the preheated oven for 25 minutes per 450g plus 25 minutes for medium, or 30 minutes per 450g plus 25 minutes for well done. Baste occasionally with dripping if desired. Remove from the oven and transfer to a serving plate. Slice, serve and enjoy.
Have your butcher bone and trim the lamb breast before you begin.