The lamb
Slash leg and stuff with rosemary and garlic. Roast on a trivet of sliced onions.
The gravy
Remove joint, cover, and put to one side to rest. With the roast tin on the hob, cook out some white wine, add hot stock. Add chopped leeks and cook until tender.
The potatoes
Parboil new potatoes with skins on, rub with salt, garlic, and oil, and put in the bottom of the oven with the lamb for the last fifty minutes of cooking.
Roast the chicken on a layer of halved white onions. When cooked, run knife through the roasted onions and transfer to pan. Add Dijon mustard, pepper, cider vinegar, chicken stock, and a glass of Reisling. Cook for a few minutes, then add the halved oyster mushrooms.