Roasted Chicken and Celery Waldorf Salad

Name: Roasted Chicken and Celery Waldorf Salad
Ingredients: 1½ pounds boneless, skinless chicken thighs (about 6); 1 cup seedless grapes; 3 stalks celery, thinly sliced, plus leaves for serving; 3 thyme springs; ¼ to ½ teaspoon crushed red pepper; 3 tablespoons olive oil; Kosher salt and black pepper; ¼ cup plain yogurt; 2 tablespoons Dijon mustard; 1 tablespoon apple cider vinegar; 1 large head escarole, torn into bite-size pieces (about 12 cups); ½ cup toasted walnuts, roughly chopped; Baguette, for serving