Melt butter in Dutch oven or heavy pot over medium; cook onion, celery, and carrots 5-7 minutes until starting to soften.
Sprinkle in flour; stir to coat and cook 2 minutes, stirring constantly to prevent burning.
Gradually stir in broth to prevent lumps. Add potatoes, thyme, and bay; boil, reduce heat, and simmer 15 minutes until potatoes tender.
Meanwhile, whisk dumpling flour, baking powder, and salt in medium bowl. Stir in milk and melted butter just until combined; do not overmix.
Add turkey; season with salt and pepper. Remove bay.
Drop heaping tablespoons batter onto simmering stew, about 8-10 dumplings. Cover tightly; cook 15 minutes without lifting lid.
Dumplings should be fluffy with clean toothpick. Serve immediately in large bowls.