Prepare the chicken: If using leftover or rotisserie chicken, shred or chop it into bite-sized pieces. Add to a large mixing bowl. If using chicken thighs, add to baking sheet and drizzle with olive oil and season with salt and pepper. cook on 375F for 20 minutes. When cool enough to touch, shred or chop.
Prep the vegetables and pear: Finely dice the celery and fennel, then dice the pear into small pieces. Add everything to the bowl with the chicken.
Make the maple Dijon dressing: In a small bowl, whisk together the mayo, red wine vinegar, maple syrup, and Dijon mustard until smooth. Slowly drizzle in the olive oil while whisking until the dressing is creamy and combined. Season with salt and pepper to taste.
Assemble the chicken salad: Add the chopped walnuts and crumbled blue cheese to the chicken mixture. Pour the maple Dijon dressing over the top and gently toss until everything is evenly coated.
Make sandwiches: Toast bread and add scoops of chicken salad. Top with a handful of arugula. Slice and enjoy!