Melt 28 g butter in a 6-quart / 5.7 L Dutch oven (28 cm / 11-inch) over medium heat. Add the onion, carrots and celery and cook for 7 to 8 minutes, stirring, until the onion is soft and translucent. Stir in the garlic for 30 seconds, then sprinkle over 50 g flour and stir for 1 minute until no dry streaks remain.
Whisk in the broth a ladleful at a time so the flour dissolves without lumps. Add the thyme, bay leaf, 1 1/2 tsp salt, pepper and chicken thighs. Bring to a gentle boil, drop the heat to low, partly cover, and simmer for 20 minutes, until the thickest thigh reads 165 F / 74 C. The bubbles should barely break the surface.
While the chicken simmers, whisk 240 g flour, the baking powder and 1/2 tsp salt in a 3-quart / 2.8 L bowl. Cut in 56 g cold butter with a pastry cutter or fork until it looks like pea-sized crumbs. Add the cold plain nonfat Greek yogurt and stir with a fork about 10 strokes, until a sticky, shaggy dough forms. Add 1 to 2 tbsp milk only if dry flour pockets remain.
Lift the chicken onto a plate, fish out the bay leaf, and shred the thighs with two forks. Return the chicken to the pot with the milk and frozen peas. Bring back to a gentle simmer for 3 to 4 minutes, until the stew coats the back of a spoon. Taste and add salt if needed.
Drop the dough in 12 heaping 3-tablespoon scoops (about 50 g each) over the surface of the stew, leaving small gaps. Cover tightly and simmer on low for 15 minutes without lifting the lid for the first 12. The dumplings are done when they're firm and puffed and a toothpick comes out clean. Ladle into bowls, 2 dumplings each, and top with parsley and black pepper.