This deliciously rich gratin is Robert Stehling's take on the Southern casseroles he refers to as "church-supper dishes." Slideshow: More Delicious Gratins
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.
The gratin can be baked for 50 minutes, then cooled and refrigerated overnight. Bring the gratin to room temperature and bake in a 400° oven for 15 minutes, or until the limas are tender, before adding the crumb topping and broiling.