Add the dressing ingredients to a mason jar. Shake well. Set aside.
In a large mixing bowl, add the arugula and most of the red onions, walnuts, and cranberries (reserving some of each for the top of the salad). Drizzle half of the dressing over the salad and toss to coat.
Transfer the salad to a serving platter or shallow bowl. Arrange the apple slices on the salad. Add the rest of the onions, walnuts, and cranberries. Add the blue cheese. Drizzle with more dressing or serve the extra on the side.
It's best to assemble this salad shortly before serving. Add the apples and the vinaigrette right before serving. I like to dress the salad with half of the dressing and serve the remaining on the side.
If you need to prepare this ahead of time, combine everything except the apples and the dressing. Arrange on a serving platter and loosely cover with plastic wrap and store in the refrigerator. Then, right before serving, arrange the apples on top and add the vinaigrette.
Toast the nuts before assembling the salad. Toasting enhances their flavor and it only takes a few minutes. Place the nuts in a non-stick skillet set over medium-low heat. Warm until the nuts are fragrant. If you have a few extra minutes, this step is worthwhile.