Add leftover shredded chicken to a mixing bowl.
Chop onion and slice 1/2 cup of grapes in half; add to chicken. Mix in cranberries, almonds, yogurt, lemon juice, garlic powder, salt, and pepper; mix well to combine. (If it appears too dry, add more yogurt).
Slice open rolls and pinch the center of each roll with your fingers, pull out the portion of bread grasped to make a well for the chicken mixture. Top rolls with chicken and spinach, close sandwich, and slice in half.
Remove seeds from bell pepper and cut into strips. Serve sandwiches with remaining grapes and pepper slices on the side.