Whisk egg whites until soft peaks form in a stand mixer. Add salt and beat until stiff peaks form.
In a separate bowl whisk together egg yolks, scallion and dill. Gently fold the egg whites into the egg yolk mixture.
Fold matzo meal into this egg mixture.
Let matzo dough sit for 5 to 10 minutes.
In a wide saucepan bring 3 inches of water to a boil. Form the matzo dough into 12 balls. Drop them into the boiling water, cover, and simmer for 25 minutes. Remove with a slotted spoon. Set aside.
In a saucepan heat the butter and oil over moderately low heat. Sweat the leeks and garlic for about 6 minutes or until soft but not browned.
Add the herbs and the lemon juice and stir. Add the stock and bring almost to a boil.
Puree the soup in batches in a blender. Pass half the pureed soup through a sieve. Return soup to the pot. Stir in the salt and pepper and set aside.
Place 3 matzo balls in each of 4 bowls and ladle in the broth. Garnish with chives.