Mix all the ingredients for homemade meatballs in a large bowl. Form 24 meatballs.
Add oil to a large skillet on medium-high heat. Once the oil starts to shimmer, add in meatballs. Turn the meatballs every minute or so to brown the outside.
Once the meatballs are brown, turn the heat to low and slowly add the marinara sauce. Reduce the heat to medium-low, cover the skillet with a lid, and cook for 15 minutes, or until the internal temperature is 160F.
If You're Using Frozen Meatballs
Heat the marinara sauce in a large skillet over medium heat. Once the sauce starts to simmer, add in the meatballs and cover. Reduce the heat to medium-low and cook until the meatballs are warm, about 25 minutes.
To Make the Meatball Subs
While the meatballs are cooking, place the buns on a baking sheet and position the top rack in your oven 6" inches below the broiler.
Preheat the broiler. Add 3 meatballs to each roll. Top with marinara sauce and shredded mozzarella. Set the baking sheet under the broiler and cook until the cheese melts, about 2-3 minutes. Be careful the buns don't burn.