METHOD FOR THE MEATBALLS: Mix all the meatball ingredients together except the sauce, being careful not to overwork. Using wet hands roll the meatball mixture into little balls, around 20g each so you end up with around 24 meatballs. Gently fry the meatballs in a non-stick pan until nicely browned. Pour over the tomato sauce, cover with a lid and simmer gently for 45 minutes.
METHOD FOR THE CHEESE SAUCE: Melt the butter in a saucepan, add flour and cook for a minute over low heat. Increase heat to medium and slowly add the milk whisking to a smooth sauce. Fold through the grated cheese until smooth and add the mustard, season to taste. BUILDING THE SUB: Reheat the baguettes. Cut the baguettes into half through the centre ready to fill. Spoon the meatballs and a little tomato sauce into the baguette. Top with the warm cheese sauce, basil leaves and serve straight away.