Put the chicken, matzo ball mix, onion, dill, parsley, butter, egg, salt, and pepper in a large bowl.
Use your hands to mix the ingredients until well combined.
Bring the chicken broth, carrots, and celery to a simmer in a large pot.
While the chicken broth is heating up, use your hands or a cookie scoop to form into 2-inch balls.
Put the meatballs in the chicken broth as you make them.
Simmer the meatballs for 5 minutes.
Serve in bowls as a soup topped with dill and celery leaves.