Preheat oven to 400°F.
Soak white bread slices in 1/2 cup milk, set aside.
In a small skillet heat oil over medium heat. Add garlic and onion. Sauté until softened, set aside.
In a large bowl, combine Italian sausage, ground turkey, egg and Parmesan cheese. Add onion mixture and mix to combine. Strain bread slices and add to the ground meat mix.
Using your hands, mix all the ingredients together just until incorporated. Do not over mix. Shape meatball mixture into 2" meatballs. Place meatballs on a cooking sheet that has been sprayed with cooking spray. Bake in preheated oven for 20 minutes.
Turn meatballs and cook an additional 5-10 minutes. Remove and allow to cool some. Preheat oven to broil. Place a large spoonful of sauce on bottoms of each bun.
Top each bun evenly with slices of cheese. Add 3-4 meatballs over the cheese and spread with more sauce. Broil for 1-2 minutes or until cheese has melted.