Note that it's tricky to find dried chipotle chilis (or any dried chilis) with a Kosher for Passover certification. There's nothing inherently not kosher for Passover about dried chipotle, but sadly no one makes one with a Passover-friendly hekhsher. —Leah Koenig
6 to 8
whole chicken (about 4 pounds)
large carrots, peeled and halved crosswise
parsnip, peeled and halved crosswise
stalks celery, halved crosswise
large white onion, peeled, and halved through the root
medium leeks (white and light green parts), end trimmed, halved through the root, and rinsed
garlic cloves, peeled and smashed
dried chipotle chili, split lengthwise and seeds removed
Place the chicken, carrots, parsnip, celery, onion, leeks, garlic, bay leaf, peppercorns, chipotle chili, and cilantro in a large soup pot and cover with cold water by 1 inch. Bring to a low boil over high heat, then reduce the heat to low. Gently simmer, partially covered, skimming off any foam that accumulates, until the chicken is very tender and falling off the bone, about 1 1/2 hours. The soup should roll along at a very gentle simmer. If it starts to bubble too vigorously, turn the heat down.
Remove the chicken and vegetables from the pot, transfer to a cutting board and let cool to the touch. Strain the broth through a fine-mesh sieve into a large bowl. Return the strained broth back to the pot. Discard the onion, leeks, garlic, bay leaf, peppercorns, chili, and cilantro. Chop the carrots, parsnip, and celery into bite-size pieces and return to the pot.
Using your fingers, remove all the meat from the bones and roughly chop and return to the pot. Discard the chicken skin and bones. Stir in 1 tablespoon salt and a generous amount of black pepper. Taste and adjust seasoning, if desired. Keep warm.
Make the matzo balls: Mix together the eggs, vegetable oil, 1 teaspoon salt, matzo meal, parsley, if using, and seltzer in a bowl. Cover and refrigerate for 30 minutes.
Meanwhile, bring a large pot of salted water to a boil over high heat. Lower heat to medium-low and keep at a simmer.
Moisten your hands with water; scoop out a rounded tablespoon of matzo ball batter and roll it into a 1-inch ball. Drop into water, and repeat with remaining batter. Cover the pot, and simmer until matzo balls are tender and puffed, 30-35 minutes. (If you cut one in half, it should be uniformly pale in color throughout.)
To serve: Place a few matzo balls into serving bowls and ladle soup over top. Serve hot with lime wedges, white onion, cilantro leaves, and avocado on the side for topping.