Preheat the oven to 325°F (160°C). Spread the walnuts in a small baking pan and toast for about 10 minutes until fragrant; remove and let cool.
Increase oven temperature to 375°F (190°C). Place the chicken breast (with skin and bone) in a small casserole dish and sprinkle evenly with paprika.
Bake the chicken for about 15–20 minutes, or until the juices run clear and the internal temperature reaches 165°F (74°C). Remove and let cool until safe to handle.
Remove the chicken skin and bones, then cut the cooked chicken into bite-size pieces.
In a medium bowl, combine the chopped bok choy, diced celery, cooled toasted walnuts, quartered red grapes, black pepper, and salt.
Add the chicken pieces and mayonnaise to the bowl and toss gently until everything is evenly coated.
Lightly toast the whole grain bread if desired. Divide the chicken salad between two sandwiches (two slices each) or serve wrapped in lettuce leaves.
Cut each sandwich in half and serve immediately.