To make meatballs: Preheat oven to 400°F. Spray a broiler pan or a large baking pan fitted with a rack with cooking spray. Set aside.
To a medium size mixing bowl add ground beef and sausage, onion, panko, Parmesan cheese, Italian seasoning, 1 teaspoon salt, and red pepper flakes. Beat milk and eggs together then, add to the bowl. Using your hands, mix everything together thoroughly.
Form into 1 - 1 ½ inch meatballs and place onto the pan without touching. (It's helpful to use a medium size scoop to keep meatballs uniform in size) Bake for 22-25 minutes.
To assemble subs: Preheat oven to broil. Line a large baking pan with parchment. Mix ½ teaspoon garlic salt into butter. Spread on each side of bread. Toast very lightly then remove from oven. Lower oven temp to 425°F.
Arrange 5 cheese slices on bottom half. Toss meatballs with half jar of marinara sauce. Arrange over cheese.
Top meatballs with remaining cheese slices. Sprinkle with additional Parmesan cheese, eyeball the amount. Fold top half over meatballs, leaving cheese partially exposed. (You can use a 6 inch water soaked bamboo skewer to hold the two sides together, if needed)
Lay foil loosely on top. Bake for 15 minutes. Remove foil and bake for an additional 5-10 minutes. Increase to broil at the end of baking to brown cheese.
Use a sharp knife to cut into 4 even pieces, or as desired.
Sprinkle with fresh parsley and serve with remaining warmed marinara on the side.