Heat the oven to Gas Mark 7, 220°C (425°F).
To make the meatballs: Put the mince, onion, garlic, egg, breadcrumbs, 1 tablesp., Dijon mustard and parsley in a bowl.  Season with a little salt and black pepper. Mix well, then shape into 18-24 meatballs.
To cook the meatballs: Put the meatballs, in a single layer, in a roasting tin and drizzle over a little olive oil.  Toss gently. Place in the oven and cook about 15 minutes until golden brown. While the meatballs are cooking make the sauce. 
To make the sauce: Heat a tablespoon of oil in a large non-stick frying pan over a medium heat. Add the leek and onion and cook for 6-8 minutes until they soften and are turning golden. Stir frequently. Then add the wine and cook for 1 minute to reduce a little. Add the stock and cream increase the heat and simmer for 5 minutes. Add the mustard and parsley, season and taste. Take the meatballs from the oven, pour over the sauce then return the roasting tin to the oven and cook for another 10 minutes. 
To serve: Serve with brown rice.