Optional: In a skillet sprayed with cooking spray, sauté the diced celery, carrots, and onion until softened. Season with salt, pepper, and mustard powder.
Add the sautéed (or raw) vegetables, raw chicken breasts, water, bay leaf, and Better Than Bouillon to the slow cooker. Stir slightly.
Cover and cook on LOW for 6-7 hours (or HIGH for 3-4 hours).
Once cooked, remove the chicken breasts. Turn the slow cooker to HIGH and quickly replace the lid. Shred the chicken and return it to the slow cooker.
In a bowl, whisk the cold milk and cornstarch until smooth. Pour this slurry into the slow cooker, stir well, and replace the lid.
Make the dumplings: In a medium bowl, whisk the flour, baking powder, seasoned salt, Italian seasoning, and shredded cheese. Slowly stir in the milk and melted butter until a sticky dough forms. Do not overmix.
Spray a teaspoon measuring spoon with cooking spray. Scoop small, teaspoon-sized mounds of dough and drop them into the slow cooker.
Cover tightly and cook on HIGH for exactly 1 hour. Do not lift the lid during this time!
After 1 hour, check seasonings, serve hot, and enjoy.