Preheat the oven to 180C/350F/Gas 4.
Cover the grouse breasts with strips of pancetta to protect them from the heat and roast the bird whole on the bone for nine minutes or until cooked through (depending on size).
When you think the grouse is cooked, test by piercing the thigh with a skewer at the thickest part. The juices should run clear. If not, cook for a few minutes longer. Remove the legs and the breasts, and set aside in a warm place, along with the cooked pancetta.
Chop up the grouse carcass and heat one tablespoon of the vegetable oil in a pan. Gently fry the carcass with shallots, brandy and peppercorns. Add a little chicken stock and cook to reduce it very quickly to create a concentrated grouse flavour in the jus. Strain through a sieve and set aside. This is best done just before serving.
Slice the Highland Burgundy potatoes thinly on a mandoline. Heat the remaining vegetable oil in a pan. Add the potatoes to the pan and lightly fry in vegetable oil until crisp and golden-brown. Season with salt and freshly ground black pepper.
Place the celeriac into a small pan, cover with equal amounts of milk and cream and cook until soft. Once soft, drain, discarding the milk and cream. Purée using a hand-blender until smooth. Season with salt and freshly ground black pepper, to taste.
Heat the butter in a frying pan and add the mushrooms. Gently fry the mushrooms until cooked all the way through.
To serve, arrange the grouse breasts and the leg meat on the plate and surround with spoonfuls of celeriac purée and sautéed wild mushrooms. Pour the sauce over the top of the grouse and garnish with the set-aside pancetta. Serve with some of the Highland Burgundy chips.