Put 1.8kg of whole chicken and 20 cups of water in a large stockpot and bring to a boil. Lower to a steady simmer and skim off the foam for 10-15 minutes, replacing the skimmed liquid with hot water.
Add half of 900g of celery in large pieces, 1 yellow onion, halved but unpeeled, 55g of fresh parsley, most of 45g of fresh dill, 2 teaspoons of black peppercorns, 2 bay leaves, and 1 tablespoon of sea salt.
Simmer gently for about 1½ hours. Meanwhile, for the matzo balls, mix 1 cup of matzo meal, 1 teaspoon of baking powder, ¼ teaspoon of salt, ¼ teaspoon of garlic powder, and ¼ teaspoon of onion powder.
Beat 4 large eggs with ¼ cup of melted schmaltz (chicken fat). Stir in the dry ingredients until just combined and refrigerate for 30 minutes.
When the chicken is tender, lift it out and let it and the broth cool for 20-30 minutes. Strain the broth, discard the solids, and return the broth to the pot.
Bring the broth back to a simmer and roll the matzo mixture into 1in / 2.5cm balls. Drop them in, cover, and simmer for 30 minutes without lifting the lid.
Add the remaining celery, sliced, and 450g of sliced carrots. Cover and simmer for 20 minutes, until the vegetables are tender and a halved matzo ball is no darker in the center than at the edges.
Pull the chicken meat into bite-size pieces, discarding the skin and bones. Stir it into the soup with the remaining dill, chopped, and simmer for 2 minutes. Season with salt.
