Remove the meat from any bone, skin, and fat. Cut into ¾" thick slices. Place the slices in a large, dep pan (at least 9" wide). Add onion, celery, carrots, bay leaves, and a smattering of pepper to the pan. Cover with 12 cups of water and bring to a boil over high heat. Put on the lid, then turn heat down to a simmer. Let cook for 1½ hours, skimmy foam every now and again.
While this is simmering, make the dumplings. Place flour in a bowl with baking powder and a pinch of salt. Using a box grater, grate your cold butter into the bowl along with a few good grates of nutmeg. Add about ¼ cup of cold water and then, using your hand, work everything into a ball of dough. It should be firm and fairly malleable. Add another drizzle or two more water if it seems to dry. Work quickly so that your body heat doesn't melt all the butter.
Dust a tray with flour and then divide the dough into 12 fairly equal pieces, roll into balls, and place on tray. Refrigerate until needed.
When cooking time is up, either pull your meat into shreds or pieces using a couple of forks or remove to a cutting board and use a knife. Return to pot. Bring back to a boil then add the peas, rosemary, and lemon zest. Plop in the reserved dumpling dough. Turn heat down to medium and allow to simmer for another 15-20 minutes to allow the dumplings to plump up and cook through.