Combine beef and pork mince and mix with your fingers 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. This will help them hold their shape whilst cooking.
In a frying pan, heat oil on 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 (180°C or 160°C with a fan) and cook for 30 minutes more.
Melt 40g of butter in a pan. Whisk in 40g of flour and stir for 2 minutes. Add 150ml of veg stock and 150ml of beef stock; continue to stir. Add 150ml of double cream, 2 tsp soy sauce, and 1 tsp dijon mustard.