Combine pork, beef, eggs, panko, salt, pepper, and nutmeg in a large bowl. When the onions are slightly cooled, add to the mix then use your hands to mix everything throughly.
Shape into 1 inch meatballs, being sure to tightly roll them around so they keep their round shape. Place meatballs on a plate or tray.
Over medium heat, melt 1-2 tablespoons of butter in the same pan that you cooked the onions in. When hot, add meatballs to the pan, cooking in batches without overcrowding. Immediately shake the pan as you add a batch of meatballs to ensure the balls roll around and maintain their meatball shape.
Fry, shaking the pan every so often, until meatballs are golden brown and cooked through. Remove the meatballs from the pan as they cook.
When all the meatballs are cooked, make the sauce: Add 2 tablespoons of butter to the pan and melt over medium heat. Sprinkle on the flour, whisk and cook for 1-2 minutes. Slowly whisk in the beef stock and cream and cook, whisking constantly, until the sauce starts to thicken. Taste and season with salt, pepper, and soy sauce if using.
