Add 2 cups shredded rotisserie chicken to a mixing bowl.
Stir in 1/2 cup cottage cheese, 3 tbsp Buffalo sauce, and 1 tbsp Kewpie mayo until the chicken is evenly coated.
Fold in red onion, celery, and grapes. Season with salt and black pepper to taste.
Layer spinach on 2 slices of toasted sourdough. Pile the Buffalo chicken mixture on top.
Close with the remaining bread slices and serve warm or room temp.