Whisk the mayonnaise together with the Greek yogurt, apple cider vinegar, kosher salt, and black pepper. Allow the dressing to rest while preparing the vegetables.
Run the Brussels sprouts through the food processor, using the slicing blade. Transfer the shredded sprouts to the serving bowl, and toss with the apples, celery, and almonds. Drizzle the dressing over the salad and toss to coat, then serve.