Start by heating the butter and olive oil in a skillet over medium heat. Once the butter has melted and the mixture is hot, add the chopped onion. Sauté for about 5 minutes, or until the onion is translucent and fragrant.
In a large mixing bowl, combine the ground chicken, sautéed onions, breadcrumbs, milk, egg, chopped parsley, minced garlic, salt, and pepper. Mix everything gently until just combined.
With clean hands, scoop out about 2 tablespoons of the mixture and roll it into a ball. Place the formed meatballs on a plate or baking sheet.
In a large skillet, heat about ¼ inch of vegetable oil over medium-high heat. Once the oil is hot, add the meatballs, frying them for about 4-5 minutes on each side, or until golden brown and cooked through.
Once browned, transfer the meatballs to a baking sheet and keep warm in a preheated oven at 200°F. In the same skillet, pour in the marinara sauce and let the meatballs simmer for about 10 minutes.