Shredded sharp white cheddarMonterey jack, or swiss cheese (optional)
To prepare the shredded chicken in the slow cooker: Arrange the breasts in a single layer in a 6-quart or larger slow cooker. Sprinkle with salt and pepper. Cover and cook on LOW for 1 1/2 to 2 hours, until the chicken reaches 165 degrees F on an instant read thermometer (if using breasts of a different size or to swap chicken thighs, see my post for Crock Pot Shredded Chicken for cooking times and tips). You also can cook and shred the chicken breasts in advance (see recipe notes for fast, easy methods).
While the chicken cooks, melt the butter in a medium saucepan over medium heat. Sprinkle the flour over the top. Whisk to combine, then continue whisking and cooking until the flour turns golden brown and smells slightly nutty, at least one full minute. Make sure you completely cook the flour or it will have a raw taste in the final dish.
Gradually add the milk and broth, whisking all the while to smooth out any lumps. Stir in the 1/4 teaspoon kosher salt, 1/4 teaspoon black pepper, Parmesan, poultry seasoning, Italian seasoning, and garlic powder. Continue cooking, stirring constantly, until the mixture thickens a bit more, about 3 to 5 minutes depending upon the size of your pan. Remove from the heat.
Deeply toast the bread, either in your toaster, or for 8 to 10 minutes in a 350 degree F oven, turning it over once halfway through. It should be deep golden brown, dry, and just on the verge of being burned without crossing the line. Let cool.
Once the chicken has cooked, transfer it to a bowl. Discard any remaining cooking liquid and, with a paper towel, carefully wipe the slow cooker clean. Shred the chicken with two forks, your fingers, or (for max ease) use a hand mixer and shred it right in the crock pot. If you are using shredded chicken that’s been cooked in advance, add it to the slow cooker now.
Pour the thickened sauce from the pan over the shredded chicken. Tear and crumble the toasted bread into small pieces right over the top (you also can pulse it into crumbs in a food processor, but I find my fingers just as easy). Add the Greek yogurt. With a large spoon, stir to combine.
Recover the crockpot and let cook on low for 1 hour. Just before serving, uncover and give the chicken a big stir.
To serve as sandwiches, pile the chicken onto buns and top with cheese. Or, serve it open faced over toasted bread, with the cheese on top (you can pop the bread under the broiler for a few moments to brown the cheese if you like). Enjoy!