Pre-heat oven to 350 degrees. Place diced chicken breast on baking sheet and put in oven. Cook for about 20 minutes, or until chicken has reached 165 degrees internally.
Combine celery, green onion, mayonnaise, Dijon mustard and lemon juice in a bowl. Once chicken cools slightly, place chicken in the bowl with combined ingredients and mix until chicken is coated.
Serve on whole-wheat bread as a sandwich, or with sliced vegetables such as cucumbers and bell peppers.