Mix the ground beef, breadcrumbs, egg, minced garlic, Italian seasoning, salt, and pepper in a large bowl until just combined. Form the mixture into 16 meatballs about 1 1/2 inches wide.
Heat the olive oil in a large skillet over medium heat. Add the meatballs and cook, turning occasionally, until browned on all sides and cooked through, about 10 minutes.
Pour the marinara sauce into the skillet with the meatballs. Reduce the heat to low and let everything simmer together for 5 minutes so the meatballs absorb some sauce.
Split the sub rolls and place them on a baking sheet. Spoon several meatballs and sauce into each roll. Lay slices of mozzarella over the top of the meatballs.
Broil the assembled subs for 2 to 3 minutes until the cheese melts and begins to bubble. Remove from the oven and serve right away.