Season the chicken thighs with a pinch of salt and pepper. If you like a deeper flavor, you can dredge them lightly in a spoonful of flour before browning, but this step is optional since the slow cooker will do the work.
Place the diced onion, sliced carrots, and celery in the bottom of the crockpot. Add minced garlic, thyme, sage (if using), bay leaves, salt, and pepper. Pour in the chicken broth and water, giving everything a gentle stir to mix the aromatics.
Nestle the seasoned chicken thighs into the liquid. Cover and cook on low for 6-7 hours, or on high for 3-4 hours, until the chicken is tender and easily shreds with a fork.
About 25 minutes before the end of cooking, make the dumpling batter. In a bowl, whisk together flour, baking powder, and salt. Stir in milk and melted butter just until the batter comes together; it should be slightly thick.
If you’d like to add peas, stir them into the pot now. If the broth is very thin, you can remove a cup of liquid, whisk in a tablespoon or two of flour, and return it to the pot to thicken (or simply rely on the dumplings to thicken as they cook).
Carefully remove the chicken from the crockpot and shred it with two forks. Return the shredded chicken to the pot, stirring to combine.
Drop spoonfuls of the dumpling batter onto the surface of the hot soup in the crockpot. Cover and cook on high for about 20-25 minutes, until the dumplings are cooked through and fluffy. Avoid lifting the lid too often, or the dumplings may not rise properly.
Taste and adjust seasoning with salt and pepper. Remove bay leaves before serving.
Serve hot in bowls, with a ladle of the creamy broth and several dumplings per serving. A simple side of crusty bread or a light salad complements the meal nicely.