To tenderize mustard greens and mellow their fiery flavor, dress the raw greens about 15 minutes before serving. If you prefer them spicy, drizzle the dressing at the table, as we suggest in the recipe.
Preheat oven to 350°. Arrange bread cubes in a single layer in a jelly-roll pan. Place bacon slices over bread cubes, so that most of bread is covered. Bake 25 to 30 minutes or until bacon and bread are crisp. Meanwhile, stir together shallot and next 3 ingredients in a medium bowl. Let stand 10 minutes. Whisk in yogurt, oil, and grated cheese. Crumble bacon; toss bacon and croutons with greens. Arrange salad on a platter. Drizzle 3 to 4 Tbsp. dressing over salad. Sprinkle with salt and pepper. Serve immediately with remaining dressing.