1Heat a large pot over medium-high heat. Season chicken pieces with salt and pepper, then add to the pot and cook until lightly browned on all sides, about 5 minutes. Transfer to a plate.
2In the same pot, add diced onion, celery, and carrot. Sauté for 4 minutes until softened. Stir in minced garlic and cook for 1 minute more.
3Pour in chicken broth, scraping up any browned bits from the bottom. Return chicken to the pot and add thyme and bay leaf. Bring to a simmer, then reduce heat to medium and cook for 15 minutes until chicken is cooked through.
4While the broth simmers, make the dumpling dough: whisk together 2 cup flour, baking powder, and 0.5 tsp salt in a bowl. Cut 6 tbsp cold butter into small cubes and work into the flour with your fingertips until it resembles coarse crumbs. Add milk a little at a time, stirring gently until just combined into a slightly sticky dough. Don't overmix.
5In a small bowl, whisk together 3 tbsp butter and 3 tbsp flour to make a paste. Stir this into the simmering broth to thicken it slightly, then stir in heavy cream. Taste and adjust seasoning.
6Drop spoonfuls of dumpling dough directly onto the surface of the simmering stew—don't stir them in. Cover the pot and simmer for 12 to 15 minutes until the dumplings are cooked through and fluffy. The liquid should be gently bubbling around the edges but not rolling.
7Remove bay leaf, taste again, and serve hot in bowls with dumplings and plenty of broth.