1) Make the matzo ball batter
In a medium bowl, whisk the eggs. Whisk in the melted schmaltz (or oil), seltzer, salt, pepper, and dill (if using). Add the matzo meal and stir just until combined. Do not overmix.
Cover and refrigerate for 30 minutes (or up to overnight). The chill time is not optional if you want fluffy kneidlach.
2) Start the soup base
Pro workflow tip: During the chill, fill your second pot with water for the matzo balls so it is ready to heat.
Heat olive oil in a large soup pot over medium heat. Add onion, carrots, and celery with a pinch of salt. Cook for 6 to 8 minutes, stirring occasionally, until the onions soften and everything smells like dinner.
Add garlic and cook for 30 seconds. Pour in the chicken stock, then add bay leaf and turmeric (if using). Bring to a gentle simmer, then keep it at a gentle simmer for 15 minutes so the veggies soften and the broth picks up flavor.
3) Cook the matzo balls
Before you start shaping: bring the second pot of water to a gentle boil, then reduce to a low simmer. Salt it well, like pasta water, about 1 to 2 tablespoons kosher salt for a large pot.
With wet hands, gently form the chilled batter into balls about 1 1/4 inches wide (they will expand). Try not to compress them. You will get about 12 to 16 matzo balls, depending on size.
Drop them into the simmering water. Cover and cook for 30 minutes at a steady low simmer. No peeking for the first 20 minutes if you can help it.
4) Finish the soup
While the matzo balls cook, keep the soup at a low simmer so everything stays hot and cozy. Stir the shredded chicken into the soup and let it warm through for 3 to 5 minutes. Taste the broth and adjust with salt and pepper.
Turn off the heat and add lemon juice and fresh dill. Start with 1 tablespoon lemon, taste, then add more if you want it brighter.
5) Assemble and serve
Use a slotted spoon to transfer matzo balls into bowls. Ladle hot soup over top. Finish with more dill and black pepper.