Place chicken breasts into the bottom of a 6 to 8 quart slow cooker. Top with onion and garlic and season with salt and pepper. Pour in chicken broth,
Cover and cook on low for 6 hours, or high for 3 hours. Shred chicken with a fork.
Whisk cornstarch into milk. Pour mixture into crockpot and stir to combine.
Make the dumplings by whisking together flour, baking powder and salt in a mixing bowl. Slowly pour in milk and butter and stir to combine into a dough. Roll out on a lightly floured countertop until it's 1/8th inch thick. and cut into 1 inch by 1/2 inch strips.
Transfer dumpling strips into the slow cooker. Cover and cook on high for 1 hour, until dumplings are cooked through. Serve hot.