Combine ground pork, breadcrumbs, Parmesan cheese, minced garlic, parsley, dried oregano, salt, black pepper, and beaten egg in a large bowl. Mix well until all ingredients are evenly incorporated.
Shape the mixture into small meatballs of the desired size.
Place the meatballs in a med-high heat pan generously doused with olive oil and occasionally turn until they are browned and cooked through about 8-10 minutes.
Once cooked, remove the meatballs and set them aside.
Combine diced tomatoes, finely chopped red onion, tomato paste, balsamic vinegar, brown sugar, dried basil, salt, and pepper in a saucepan.
Cook the mixture over medium heat, stirring occasionally, until the tomatoes break down and the relish thickens, approximately 15-20 minutes.
Remove from heat and let the tomato relish cool.
Preheat your braai to medium heat.
Cut the sub rolls or baguettes lengthwise but not all the way through, creating a pocket to hold the ingredients.
Spread a generous amount of basil pesto sauce inside each roll.
Place several meatballs inside each roll and top with slices of mozzarella cheese.
Spoon tomato relish over the meatballs and cheese.
Wrap each sandwich tightly in foil, ensuring the seams are sealed to prevent leakage.
Place the foil-wrapped sandwiches on the braai grid over medium heat and cook for approximately 10-15 minutes (turning occasionally) or until the cheese is melted and the bread is heated through.
Remove the foil-wrapped sandwiches from the braai and carefully unwrap them.
Serve the delicious Pork Meatball Subs hot, and enjoy the delightful combination of juicy pork meatballs, gooey melted cheese, tangy tomato relish, and the fresh kick of pesto.