Meanwhile bring the cream, garlic and bay leaf to a simmer. Take the pan off the heat, stir in the mustard and season well. Tip the potatoes and shallot mixture into a small, deep baking dish, you want them piled up rather than spread out. Give them a stir, then strain over the cream, discarding the garlic but putting the bay leaf back into the dish. Cover with foil and cook for 20 minutes then take off the foil and cook until the top browns, about 10 minutes. Rest for 10 minutes before you eat it.