Preheat the oven to 350 degrees F (180 degrees C). Lightly grease a loaf pan.
Combine 6 cloves garlic, onion, egg, egg yolk, half and half, 2 tablespoons parsley, and ¼ cup grated Parmesan in a bowl; mix until well combined. Add ground chicken, sausage, 1 teaspoon salt, and pepper; mix until combined. Fold in breadcrumbs until evenly mixed and form into a loaf shape.
Transfer mixture into the prepared pan and sprinkle the top evenly with 1 ½ tablespoons Parmesan cheese.
Bake in the preheated oven until the internal temperature reaches 165 degrees F (73 degrees C), about 35 to 40 minutes. Let rest while the sauce is prepared.
Meanwhile, heat 1 tablespoon olive oil in a small saucepan over medium-high heat. Add remaining 2 cloves garlic and cook until fragrant, about 1 minute. Pour in wine and broth and bring to a boil. Reduce heat and simmer until reduced by about ¾ (you should have about ⅓ cup remaining).
Turn off heat and gradually whisk in cubed butter. Stir in remaining Parmesan cheese and parsley. Season with remaining salt and crushed red pepper flakes.
Slice meatloaf and serve drizzled with garlic butter sauce.
