01 - Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.
02 - In a large mixing bowl, gently combine ground turkey, breadcrumbs, Parmesan, egg, milk, minced garlic, chopped parsley, salt, black pepper, and oregano until just incorporated.
03 - Divide mixture into 12 equal portions. Flatten each in your palm, place a mozzarella cube in the center, and completely enclose cheese with turkey mixture, rolling gently into a uniform ball.
04 - Arrange stuffed meatballs evenly on the prepared baking sheet. Bake for 22 to 25 minutes, until golden brown and cooked through.
05 - While meatballs bake, heat olive oil in a medium saucepan over medium heat. Add chopped onion and cook for 3–4 minutes until softened. Stir in minced garlic and cook for 1 minute. Add crushed tomatoes, dried basil, sugar, and season to taste with salt and pepper. Simmer uncovered for 10–12 minutes, stirring occasionally.
06 - Arrange baked meatballs on plates. Spoon tomato sauce over top and garnish with extra parsley if desired. Serve hot.