How to Make It
In a medium pot of simmering salted water, cook the asparagus over moderately high heat until tender, about 4 minutes. Using a slotted spoon, transfer the asparagus to a bowl. Add the cranberry beans to the water and simmer over moderate heat until tender, about 40 minutes. Drain the beans and transfer to the bowl.
Meanwhile, in a medium enameled, cast-iron casserole, heat 1/2 tablespoon of the olive oil. Add the scallions and cook over moderate heat until barely tender, about 1 minute. Transfer the scallions to the bowl with the asparagus. Add another 1/2 tablespoon of the olive oil to the casserole. Add the mushrooms, season with salt, cover and cook over moderate heat, stirring a few times, until lightly browned and tender, about 3 minutes. Transfer the mushrooms to the bowl.
Heat the remaining 1 tablespoon of olive oil in the casserole. Add the turnips and carrots, season with salt and cook over moderate heat for 1 minute. Add 1 cup of the stock, cover and cook over low heat, stirring occasionally, for 15 minutes. Add another 1/2 cup of the stock, cover and cook until the turnips and carrots are tender, about 10 minutes longer. Add the zucchini and 1/2 cup of the stock and simmer until the zucchini is just tender, about 4 minutes. Add the remaining 1/4 cup of stock to the casserole along with the lemon zest, lemon juice and lettuce and cook, stirring, until the lettuce just wilts, about 20 seconds. Stir in the crème fraîche.
Add the asparagus, beans, scallions and mushrooms to the stew. Simmer briefly, until heated through, about 30 seconds. Add the chervil and chives and serve the stew in bowls.