In a large bowl, mix ground chicken, egg yolk, breadcrumbs, onion mixture, dried herbs, salt, pepper and 1/2 cup grated Parmesan together. Gently mix with hands. {You can use a spoon but, honestly, hands work the best}
1 pound ground chicken, 1 egg yolk, 1/3 cup panko breadcrumbs, 1 teaspoon dried basil, 1/2 teaspoon dried thyme, 1 teaspoon dried oregano, 1/2 teaspoon salt, 1/2 teaspoon black pepper, 2/3 cup grated Parmesan cheese