Preheat oven to 400 degrees F. Heat oil and butter in a heavy-bottomed skillet. Sear the lamb shanks on all sides. Remove shanks and reserve the oil and butter. Place shanks in a roasting pan with the bay leaf, celery, carrots, and onion. Bake in the preheated oven for 20 minutes. Reduce oven temperature to 350 degrees F. Season lamb shanks with salt, pepper, garlic, rosemary, and thyme. Add red wine, tomato paste, and stock to the lamb shanks. Cover with a tightly fitting lid or aluminum foil and return to oven. Braise until tender, approximately 1 hour. Remove from oven. Lift the lamb shanks out of the sauce and keep warm. Skim any fat from the stock and strain through a fine mesh strainer. In a separate pot, mix the reserved oil and butter with the flour. Add the strained braising stock, whisking slowly until a smooth consistency is achieved. Return lamb shanks to sauce and simmer for 15 miutes. Correct seasoning.