Preheat the oven to 400 degrees and line a sheet pan with parchment paper.
In a large mixing bowl, add the egg, breadcrumbs, Parmesan, Italian seasoning, minced onion, garlic powder, salt, pepper, and water, and stir together with a fork until well combined. Add the ground beef and ground turkey to the bowl and use clean hands to mix everything together until just combined.
On a clean, dry surface (like a cutting board), separate your meat mixture into 15 equal piles. Wet your hands or spray them with a bit of cooking spray and then roll each little pile into a ball. Place each ball on the prepared baking sheet. Place the baking sheet in the oven and bake for 15-20 minutes until cooked through.
When there is about 5 minutes left on the meatballs, pour the pasta sauce into a medium saucepan and cover. Place on a burner over medium-low heat, and switch to low heat if it starts bubbling. Stir occasionally.
When the meatballs are finished, combine them with the sauce (you can put them all together, or you can separate out ¼ cup of sauce to every 3 meatballs, as that’s what each sub will get). Move an oven rack to the top position and switch the oven to broil.
Remove the used parchment from your sheet pan and replace it with a clean piece. Place your open buns onto the baking sheet and fill each one with 3 meatballs covered in ¼ cup of sauce. Sprinkle 2 tablespoons of shredded Mozzarella over the top of the meatballs on each sub. Place the pan under the broiler and broil for about 2 minutes*, or until the cheese is melted and the tops of the buns are a bit toasted. *Keep an eye on the subs under the broiler, some cook faster than others and things can go from golden/toasted to blackened fairly quickly!