Preheat the oven to 325°F (163°C). Spread the walnuts on a small baking pan and toast in the oven for about 10 minutes until fragrant; remove and let cool.
Increase oven temperature to 375°F (190°C). Place the chicken breast in a small casserole dish and sprinkle evenly with the paprika.
Bake the chicken for 15–20 minutes, or until juices run clear and an instant-read thermometer registers 165°F (74°C). Let cool slightly.
Remove and discard the skin and bones from the chicken, then cut the meat 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 chopped chicken and mayonnaise to the bowl and toss gently until everything is evenly coated.
Lightly toast the whole grain bread. Divide the chicken salad between two slices each, top with the remaining slices, cut sandwiches in half, and serve (or place salad in a roll or wrap in a lettuce leaf if preferred).
