Place bacon on a microwave-safe plate lined with paper towels. Microwave at HIGH 5 minutes. Remove from microwave, and crumble bacon.
Combine buttermilk and next 4 ingredients, stirring well with a whisk.
Combine salad greens and mushrooms; arrange evenly on salad plates. Spoon 2 tablespoons buttermilk dressing over each salad; sprinkle evenly with bacon. Sprinkle with freshly ground black pepper, if desired.