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 find, substitute them for blue oysters.