Mix the ground beef, breadcrumbs, egg, minced garlic, Italian seasoning, salt, and black pepper in a bowl until just combined. Form the mixture into 8 small meatballs, each about 1.5 inches wide.

Heat the olive oil in a skillet over medium heat. Add the meatballs and cook for 8 to 10 minutes, turning them every couple of minutes until browned on all sides and cooked through.

Pour the marinara sauce into the skillet with the meatballs. Lower the heat and let everything simmer together for 5 minutes so the sauce coats the meatballs.

Split the mini brioche buns and place them cut side up on a baking sheet. Toast them under the broiler for 1 to 2 minutes until the edges turn golden.

Place one meatball on each bottom bun half. Add a half slice of mozzarella on top of each meatball so it begins to melt from the heat. Spoon a little extra sauce over the cheese, then sprinkle with chopped parsley. Add the top bun halves and serve right away.