Heat a large pot over medium heat and add the chicken pieces. Cook until lightly browned on all sides.
Add the onion, carrots, celery, potatoes, and garlic to the pot. Stir and cook for 5 minutes until the vegetables begin to soften.
Pour in the chicken broth, thyme, salt, and pepper. Bring the mixture to a simmer and cook for 15 minutes.
In a mixing bowl, combine the flour, baking powder, and a pinch of salt. Stir in the milk and melted butter until a soft dough forms.
Drop spoonfuls of the dumpling dough into the simmering stew. Cover the pot and cook for 10 minutes without lifting the lid.
Stir in the frozen peas and cook for 3 more minutes until the peas are heated through and the dumplings are cooked.