Matzo Ball Soup with Onion Butter & Dill

Ingredients

• One whole 4- to 4 ½-pound chicken
• 1 large onion, peeled and quartered
• 2 heads garlic (unpeeled), halved crosswise
• 2 celery stalks, chopped
• 1 large carrot, chopped
• 3-4 sprigs fresh parsley
• 3-4 fresh bay leaves
• 1/2  teaspoon whole black peppercorns
• Kosher salt
• 1 cup matzo meal (not matzo ball mix)
• ⅓ cup finely chopped chives
• 2 teaspoons Onion Butter seasoning, plus more for finishing
• 5 large eggs
• ⅓ cup chicken fat, vegetable oil or unsalted butter melted
• ¼ cup seltzer water
• 2 celery heart stalks thinly sliced (save any leaves for garnish)
• 1 carrot, thinly sliced into half moons
• ⅓ cup chopped dill leaves
• Freshly ground black pepper