Ultimate Matzo Ball Soup

Name: Ultimate Matzo Ball Soup
Ingredients: 1/3 cup schmaltz or duck fat; 1 small yellow onion, finely chopped (about 3/4 cup); 4 matzo sheets (about 4 3/4 ounces); 1/2 teaspoon garlic powder; 1/4 teaspoon ground white pepper; 5 1/2 teaspoons kosher salt, divided; 5 large eggs; 1/4 cup plain seltzer water; 4 quarts homemade or high-quality store-bought chicken stock, divided; 3 small carrots, thinly sliced (about 1 cup); 2 celery stalks, thinly sliced (about 1 cup); Chopped fresh dill or parsley, for garnish; Lemon wedges, for serving