How to Make It
Preheat the oven to 375°. In an 8-by-10-inch gratin dish, gently toss the potatoes with the mackerel and bay leaves. In a bowl, mix the cream with the mustard and season with salt and pepper. Pour the cream over the potatoes and bake for about 1 hour, or until the cream has thickened and the potatoes are golden and tender. Discard the bay leaves before serving.