Cook the chicken: Preheat the oven to 400°F. Place the chicken thighs on a parchment-lined baking sheet, drizzle with olive oil, and season with salt and pepper. Roast for about 20 to 25 minutes, or until the chicken reaches an internal temperature of 165°F. Let it rest for 5 to 10 minutes, then shred with two forks.
Make the dressing: In a large bowl, whisk together the mayonnaise, apple cider vinegar, honey, Dijon mustard, salt, and black pepper until smooth.
Assemble the chicken salad: Add the shredded chicken, diced apple, celery, dried cranberries, pecans, and shallot to the bowl with the dressing. Toss until everything is evenly coated.
Build the sandwiches: Layer arugula onto slices of sourdough, spoon the chicken salad over top, and add maple bacon if using. Top with another slice of sourdough and serve.