In a large bowl combine ground beef and pork with garlic, onion, egg, milk, and breadcrumbs. Season with salt and pepper and gently mix until just combined. Form into meatballs.
Heat oil in a large skillet over medium high heat. Brown the meatballs on all sides, working in batches if needed. Transfer to a plate.
In the same skillet melt butter and whisk in flour. Cook about 1 minute until lightly golden.
Slowly whisk in beef broth until smooth. Add heavy cream, Dijon mustard, and soy sauce. Season with salt and pepper.
Return meatballs to the skillet and simmer a few minutes until the sauce thickens and coats the meatballs. Serve warm.