1.Combine beef, egg, breadcrumbs, half the parmesan and the parsley in a bowl; roll tablespoons of mixture into 24 meatballs.
2.Heat oil in a large deep non-stick frying pan over medium heat; cook meatballs, turning, for 10 minutes or until browned and cooked through.
3.Add pasta sauce to pan; bring to a simmer.
4.Split rolls lengthways from the top, without cutting all the way through. Spread sides with sauce.
5.Combine rocket and vinegar in a small bowl; divide salad among rolls.
6.Fill each roll with four meatballs and scatter with the remaining parmesan.
Use gem or cos lettuce leaves instead of the rocket, if preferred.
Meatballs can be prepared to the end of step 3 and refrigerated in an airtight container up to 2 days in advance. Continue from step 4 the night before or in the morning.
Wrap subs in baking paper and pack in airtight containers. Transport in cooler bags. Refrigerate until ready to serve.