Preheat your oven to 375°F (190°C).
In a medium pot over medium heat, combine the frozen meatballs and marinara sauce. Cook until heated through, about 10–12 minutes.
While the meatballs warm up, slice the hoagie rolls lengthwise without cutting all the way through.
Optional but recommended: Lightly butter the insides and toast them in a skillet for 2–3 minutes until golden.
Fill each roll with meatballs and plenty of sauce. Top with mozzarella.
Place the subs on a baking sheet and broil for 2–3 minutes, or until the cheese is melted and bubbly.
Garnish with chopped parsley or basil if you're feeling fancy. Serve warm!