In a large bowl, combine the mayonnaise, mustard, garlic powder, and onion powder. Add the parsley, turkey, celery, corn, red bell pepper, and green onion and toss to fully coat.
Slice the tops off of the tomatoes. With a spoon, scoop out the centers of each tomato, removing the seeds and pulp. Rest the hollowed out tomatoes upside down on a paper towel for about 10 minutes while the juices drain.
Season the inside of each tomato with salt and pepper, then fill with the turkey salad mixture.