Combine turkey, oats, barbecue sauce, hickory seasoning, and granulated onion in a small bowl. Mix well and form into 1 1/2-ounce meatballs.

Preheat the oven to 350 degrees F (175 degrees C).

Pour water, Worcestershire, and tamari sauce into a 9x9-inch casserole dish. Place meatballs in the dish.

Bake in the preheated oven until no longer pink in the centers, about 45 minutes. Toast brioche buns until golden, if desired.

Serve meatballs on top of buns. Place a slice of cheese over each; top with crispy onions and barbecue sauce.