How to Make It
Preheat the oven to 400°. In a large skillet, melt 2 tablespoons of the butter. Add the sliced leeks and cook over low heat, stirring, until they are softened, about 3 minutes. Add the mushrooms, cover and cook over low heat, stirring occasionally, until they are tender, 15 minutes. Add the stock, cream, bay leaves and tarragon and bring to a boil. Season with salt and pepper and remove from the heat.
Put the frozen lima beans in the bottom of a 12- to 14-cup glass or ceramic baking dish. Pour the hot cream mixture over the limas. Bake for 1 1/4 hours, or until the cream sauce is thickened.
Preheat the broiler. In a small bowl, toss the bread crumbs with the Parmesan and melted butter. Sprinkle the crumbs over the lima beans and broil for about 1 minute, rotating the dish for even browning. Let the gratin stand for 15 minutes before serving.