Prepare the Chicken Mixture: Place the boneless, skinless chicken thighs into the slow cooker. Pour in the chicken broth, adding the diced onion, sliced carrots, diced celery, and minced garlic. Sprinkle in the dried thyme, along with salt and black pepper to taste.

Set the Slow Cooker: Cover the slow cooker with its lid and set it to cook on low for 5 hours.

Shred the Chicken: Remove the chicken thighs from the slow cooker and shred into bite-sized pieces.

Thicken the Broth: Mix all-purpose flour and cold water to create a slurry, then gradually pour it into the slow cooker.

Add the Chicken and Peas: Return the shredded chicken to the slow cooker, adding the frozen peas.

Prepare the Dumpling Batter: Whisk together the all-purpose flour, baking powder, and salt for the dumplings. Cut in the cold butter until it resembles coarse crumbs.

Combine for Dumplings: Gradually stir in the whole milk until a dough forms.

Drop the Dumplings: Drop spoonfuls of the dumpling batter evenly over the chicken mixture.

Cook the Dumplings: Cover the slow cooker and set it to high for 45 minutes.

Final Touches: Let the dish stand for about 5 minutes before serving.