Preheat the oven to 350°F.
In a large mixing bowl, combine the ground beef, breadcrumbs, minced garlic, egg, chopped parsley, and shredded Parmesan cheese.
Season the mixture with salt and black pepper.
Mix gently until all ingredients are evenly combined.
Shape the meat mixture into 24 small meatballs.
Heat the olive oil in a skillet over medium heat.
Cook the meatballs in batches for approximately 5 minutes, or until they are seared on all sides.
Arrange the seared meatballs on a parchment paper-lined baking sheet.
Bake for 12 minutes, or until the meatballs are cooked through.
Remove the meatballs from the oven and set aside.
Cut an oval shape from the top of each hoagie roll, leaving approximately 1/2 inch around the sides and ends.
Carefully hollow out the center of each roll, making sure not to cut through the bottom.
Arrange the hollowed rolls in a greased baking dish.
Place Asiago cheese slices inside the hollowed portion of each roll.
Bake the rolls for 8 minutes, or until the cheese begins to melt and the rolls are warmed.
Spoon approximately 2 tablespoons of marinara sauce into each roll.
Place 3 meatballs on top of the sauce in each roll.
Spoon the remaining marinara sauce over the meatballs.
Sprinkle additional shredded cheese over the top, if desired.
Bake for 6 to 8 minutes, or until the appetizers are hot, the sauce is warm, and the cheese is melted.
Serve warm.