Pour marinara sauce into a saucepan, cover, and heat over medium-low stirring occasionally.
Once the meatballs have finished cooking place them in the warm marinara, stir, and turn off heat.
Preheat the oven to 400°F.
Cut the baguette into 4 equal pieces and then slice in half horizontally.
Top the bottom slice of bread with the shredded mozzarella and parmesan.
Equally, divide the meatballs in sauce among the four sandwiches. You may not use all of the sauce.
Break the cheese slices in half and place over the top of the meatballs, sprinkle with 1/2 Tbsp of fresh parsley, and place the other half of the bread on top and press down gently.
Place the sandwiches on a parchment-lined baking sheet and bake for 10 minutes or until the cheese has melted and the bread is lightly golden.
Remove from oven and serve the subs with extra marinara sauce on the side.