01 - Preheat oven to 400°F.
02 - In a large skillet over medium heat, melt butter. Add diced onion, carrots, celery, and minced garlic; sauté until vegetables are tender, approximately 5 minutes.
03 - Sprinkle all-purpose flour evenly over the sautéed vegetables and stir for 1 minute to eliminate raw flour taste.
04 - Gradually whisk in chicken broth and whole milk. Cook, stirring continuously, until the mixture thickens, about 3 to 4 minutes.
05 - Fold in cooked chicken, frozen peas, salt, black pepper, and dried thyme, stirring to evenly distribute ingredients.
06 - Lightly grease a 9x13-inch casserole dish and spread the chicken mixture evenly inside.
07 - In a medium bowl, combine all-purpose flour, baking powder, and salt. Stir in melted butter and whole milk until just combined. Fold in chopped parsley if using.
08 - Using a spoon, drop portions of dumpling dough evenly across the chicken mixture.
09 - Bake uncovered for 25 to 30 minutes until dumplings are golden and cooked through.
10 - Allow casserole to cool for 5 minutes before serving for easier portioning.