Halve and peel the onion, then finely dice one of the halves. Fry it with 1/2 tbsp butter in a large pan over low-medium heat until soft & golden (try and go low and slow to tickle out the natural sweetness - about 10mins or so). Scrape out and leave to cool, then dice it as fine as you can get it. Grate the other half on a box grater and place both in a large mixing bowl.
Use a fork to stir in the breadcrumbs, cream and all of the seasoning until the breadcrumbs soak in the liquid. Stir in the egg until well-combined, then add in the pork & beef and mix well with your hand.
Use a 1 tbsp measuring spoon to scoop out 24-25 chunks of meat and place on a large tray, then roll each one into a ball (lightly wet your hands if the mix is sticky).
Heat 1 tbsp oil and butter in the pan over medium-high heat and add half of the meatballs. Swirl the pan to coat them in the fat, then continue frying and gently turning until brown all over, about 5-6mins. Remove and place in a bowl to one side and repeat with the second batch.
Pour all of the leftover fat into a bowl and place 3 tbsp of it back into the pan. Lower the heat to medium and stir in the flour to create a roux. Gradually pour in the beef stock, stirring as you go to ensure no lumps form, then stir in the cream, soy sauce, Dijon mustard and seasoning to taste (I typically go for 1/8 tsp white pepper and no added salt).
Add the meatballs into the sauce alongside any resting juices. Simmer for 5 or so mins to thicken the sauce and heat the meatballs back through, stirring occasionally. Remove from heat then serve up and enjoy!