(See NOTE below on grilled chicken)
In a large mixing bowl, gently toss together the diced chicken, celery, onion, grapes and slivered almonds.
In a small bowl make a dressing by stirring together the mayonnaise, lemon juice, mustard, dill, salt, pepper and garlic powder.
Add dressing to the chicken mixture and stir well to combine; adding more mayonnaise if needed.
Taste mixture and add additional salt or pepper, as desired.
Cover and store in refrigerator at least 3 hours before serving; best if made a day ahead.
NOTE: before grilling your boneless chicken breasts, rinse and remove any extra fat. Then, butterfly the chicken breasts (this allows for more even cooking). Generously sprinkle with salt and black pepper and let "rest" while your grill heats up. Set grill to LOW temperature and grill until done.