Most spinach-and-bacon salads include thinly sliced red onion and are served slightly wilted by a warm dressing. Once a year is enough of that sort of spinach salad for my taste. I prefer this version because the  room-temperature soy-ginger dressing is so delicious and perfectly complements not only the crisp yet tender spinach leaves but also the accompanying bacon and mushrooms. We serve this salad much more often because, just as we do, our guests always look up and smile after the first bite. To achieve just the right flavor balance, the dressing measurements yield 1½ cups, more than the salad needs; reserve it in the refrigerator for other salads, or try it instead of mayonnaise for your next cole slaw.