Combine beef and pork mince and mix thoroughly to break up any lumps. Add finely chopped onion, garlic, breadcrumbs, egg and mix. Add milk and season well with salt and pepper. Shape mixture into small, round balls. Place on a clean plate, cover and store in the fridge for 2 hours (to help them hold their shape while cooking). In a frying pan, heat oil on medium heat. When hot, gently add your meatballs and brown on all sides. When browned, add to an ovenproof dish and cover. Place in a hot oven, 350 degrees Fahrenheit or 180 degrees Celsius, and cook for a further 30 minutes.
For the Cream Sauce:
Melt the butter in a frying pan. Whisk in the plain flour and continue cooking, stirring continuously for 2 minutes. Add the vegetable stock and beef stock and continue to stir. Add the thick double cream, soy sauce and Dijon mustard. Bring to a simmer and allow the sauce to thicken. Continue to stir. To serve, spoon some sauce on the bottom of a plate. Place desired amount of meatballs on plate. Coat meatballs with more sauce.