Preheat oven to 325 degrees F.
Heat oil in deep, heavy-bottomed pot. Saute onions, mushrooms, and garlic until browned and remove/reserve. Season lamb shanks with salt and pepper and add to same pot; cook until browned on both sides. Add onion mixture and bay leaves back to pot and heat five minutes with shanks. Deglaze with wine. When most of the wine has cooked off, add one cup of stock and place in oven.
Cook for one hour, then add another cup of stock (being sure the pot is never dry). Cook another hour, then add final cup of stock. Simmer in over until meat is tender and almost falling off the bone.
Add boiled potatoes, celeriac, butter, buttermilk, salt, pepper, and roasted garlic to large mixer. Beat until smooth (do not over beat).
Serve one lamb shank per person over dollop of mashed potato/celeriac.