In a large Dutch oven or heavy pot, melt butter over medium heat. Add onion, celery, and carrots. Cook for 5-7 minutes until vegetables start to soften.
Sprinkle flour over the vegetables and stir well to coat. Cook for 2 minutes, stirring constantly to avoid burning.
Gradually pour in the broth, stirring constantly to prevent lumps. Add potatoes, thyme, and bay leaf. Bring to a boil, then reduce heat and simmer for 15 minutes until potatoes are tender.
While the stew simmers, make the dumplings. In a medium bowl, whisk together flour, baking powder, and salt. Stir in milk and melted butter just until combined (don't overmix or they'll be tough!).
Add the turkey to the stew and season with salt and pepper. Remove the bay leaf.
Drop dumpling batter by heaping tablespoonfuls onto the simmering stew (you should get about 8-10 dumplings). Cover with a tight-fitting lid and cook for 15 minutes without peeking—I know it's tempting!
Dumplings are done when they're fluffy and a toothpick inserted comes out clean. Serve immediately in big bowls and enjoy the comfort!
