Place turkey breast half in slow cooker insert. Season with salt and pepper. Pour broth around turkey; add bay leaf, thyme and oregano. Cover and cook on HIGH for 5 hours or on LOW for 7 hours.
Using a large spatula, carefully remove turkey from slow cooker to a cutting board. Remove and discard bones; strain broth from slow cooker into a bowl.
Place mashed potato granules in a medium bowl. Ladle 1 1/2 cups of the strained broth over granules. Whisk to blend, then cover and set aside.
Meanwhile, combine gravy mix and 1 cup cold water in small saucepan. Bring to a simmer; cook 1 minute.
Cut turkey crosswise into slices. Place 1 piece of bread on a plate. Spread with 1/3 cup mashed potatoes and some of the turkey, and top with a little gravy. Repeat to make 6 sandwiches and serve.
