In a 5-quart Dutch oven, melt the butter over medium heat. Add the onion, celery, carrots and garlic and cook until translucent, about 8 minutes. Do not brown. Stir in the thyme and bay leaf and cook an additional minute.
Sprinkle in the flour and stir to coat the vegetables, then slowly pour in the chicken stock, stirring constantly. Season with salt and pepper and let come to a boil. Reduce heat to medium low and cover, then simmer for 15 minutes. After 10 minutes, stir in the shredded chicken.
Meanwhile, to make your dumplings, stir together the flour, baking powder, salt, nutmeg, brown sugar, and parsley in a medium bowl. Add the butter then, using your hand, press the butter into the flour, making small crumbs of butter throughout the flour.
Pour in the buttermilk and stir together until a slightly shaggy dough forms. Do not overmix. It’s okay if a few bits of flour remain at the bottom of the bowl.
Place the dough on a floured countertop and shape into a disk then roll into a 11-inch circle, 1/8 inch thick. Using a pizza cutter, cut 1 ¼ inch wide strips on the diagonal, then turn your cutter and cut 1 ¼ inch strips the opposite direction diagonally, creating diamond shapes.
Remove the cover from the simmering broth, then drop in the dumpling strips. Stir gently, then return the cover and cook for 15 minutes, until the dough is cooked through. (To test if it’s ready, remove one dumpling from the pot, let cool, and taste. If it’s slick, light, and slightly chewy, it’s done. If not, continue cooking for another 3-5 minutes.)
Remove from the heat and stir in the lemon juice and reserved minced celery leaves.