In a large mixing bowl, add beef and pork; mix thoroughly to break up any lumps. Add finely chopped onion, garlic, breadcrumbs, and egg, and mix to combine. 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 over medium heat. When hot, gently add meatballs and brown on all sides.
When browned, add to an ovenproof dish and cover. Place in a hot oven, 350°F. or 180°C., and baked for 30 minutes.
Meanwhile, make the sauce. Melt the butter in a frying pan. Whisk in the plain flour and continue cooking, stirring continuously for 2 minutes.
Slowly add in vegetable stock and beef stock, whisking continuously. Stir in the heavy cream, soy sauce and Dijon mustard.
Bring to a simmer and allow the sauce to thicken. Continue to stir.
When ready to eat, serve with your favorite potatoes or noodles.