Fry the onion, peppers and mozzarella in oil until soft. Add garlic and butter and fry for a further 2 minutes or so. Leave the mix to cool.
Mix together the beef mince, breadcrumbs, parmesan, eggs, salt pepper and cooled duxelles mixture. Fashion into small meatballs and fry in oil.
Remove from the pan and add the butter. Heat until foaming then add the flour. Cook until it smells biscuity and has become a smooth paste.
Gradually pour in the milk, making sure it’s incorporated as you do, to make a smooth mix. Add the meatballs back in and stir in the cheese. Gently stir until melted and serve with chips or in a sub.