Saute onion in olive oil over medium heat until softened, about 8 minutes. Set aside to cool.
In a medium mixing bowl, beat egg with milk and Worcestershire sauce. Mix-in the bread crumbs, salt, pepper, sugar, cheese, parsley and spices; set aside.
Mix meats together in a large bowl, and gently mix-in the egg mixture and onion. Shape into 1" meatballs. Place meatballs on a rimmed parchment-lined baking sheet. Broil on High until lightly browned, about 10-12 minutes. Cooking time will vary depending on size of meatballs. Break one open to be sure they are cooked through, extend cooking time if needed.
Preparing the Sandwiches:
Place meatballs in marinara sauce and stir to coat and meld flavors.
Slice the rolls in a manner that will facilitate the sandwich (I slice and remove 1" of the center of the roll to minimize the amount of bread). Mince garlic and mix into butter. Spread garlic-butter on cut side of each slice of bread. Sprinkle grated cheese on top of the garlic-butter and broil on High until lightly toasted. Watching closely to avoid over browning.
Arrange meatballs on one slice of the toasted bread as desired; cover with 2-3 tablespoons of sauce and top with a slice of provolone cheese.Place the open sandwich under the broiler to melt the cheese. Top with the other side of garlic-cheese toast and enjoy.
The bread and meatballs can be made ahead of time and frozen for a quick, easy meal any night of the week.
Meatballs can be frozen and used as needed. The hot Italian sausage imparts a subtle spice, but can be substituted with regular Italian sausage.