Spinach and Blue Oyster Mushroom Salad with Bacon and Walnuts

Tiny firm mushrooms, which give a springy feel to a dish, are cooked in a different way from the soft ones in the preceding salad. For this update of the American mushroom-bacon-spinach classic, satiny little oysters, thick bacon dice, toasted nuts, and mini-greens are tossed in a brisk mustard-balsamic dressing. Trimmed and cleaned small-leaf spinach is sold loose or in bags. Or buy bunched medium leaves, trim well, and cut into small pieces. If beech mushrooms (honshimeji) are easier to f