How to Make It
In a large soup pot, cover the cranberry beans with 3 quarts of cold water. Add 1 tablespoon of kosher salt, the bundle of thyme sprigs and the bay leaves. Bring to a simmer and cook over moderate heat until the beans are tender, about 40 minutes. Drain the beans and discard the thyme sprigs and bay leaves.
Meanwhile, in a large bowl, pour 4 cups of boiling water over the dried porcini and let stand until the mushrooms are soft, about 15 minutes. Remove the mushrooms and squeeze them dry over the bowl; reserve the soaking liquid. Coarsely chop the mushrooms.
Preheat the oven to 375°. Position a rack in the top third of the oven. Slowly pour the mushroom soaking liquid into a large saucepan, discarding the last bit of gritty liquid. Add the porcini, leeks, butter and 2 teaspoons of salt. Bring to a simmer and cook over moderate heat until the leeks are very tender and the liquid has thickened slightly, about 35 minutes. Remove from the heat. Stir in the crème fraîche, 2 teaspoons of the chopped thyme, 2 teaspoons of the chopped marjoram and the reserved beans. Season with salt and pepper. Transfer the bean mixture to a 9-by-13-inch baking dish.
In a small bowl, toss the panko with the olive oil and the remaining 1 teaspoon each of chopped thyme and marjoram; season with salt and pepper. Sprinkle the panko over the beans. Bake for about 40 minutes, until the beans are bubbling and the panko is browned. Remove the cassoulet from the oven and let stand for 5 minutes. Spoon into warm bowls and serve.