In a small bowl, combine the gluten free panko breadcrumbs and milk. Let it soak for 5 minutes until the breadcrumbs absorb the liquid.
In a large bowl, combine the ground beef, ground pork, soaked breadcrumbs, egg, grated onion, garlic powder, allspice, nutmeg, salt, and black pepper. Mix gently with your hands until just combined. Roll the mixture into about 24 meatballs, roughly 1 1/2 inches each.
Heat the olive oil in a large skillet over medium-high heat. Brown the meatballs in batches, turning them every minute or so, for about 5 to 6 minutes total until golden on all sides. They do not need to be cooked through. Transfer to a plate.
In the same skillet, reduce heat to medium and add the butter. Once melted, sprinkle in the gluten free flour blend and whisk constantly for about 1 minute to cook out the raw flour taste.
Slowly pour in the beef broth while whisking continuously to prevent lumps. Add the heavy cream, tamari, and dijon mustard. Stir well and bring the sauce to a gentle simmer.
Return the meatballs to the skillet, nestling them into the sauce. Cover and simmer on low for 15 to 18 minutes, stirring gently once or twice, until the meatballs are cooked through and the sauce has thickened.
Taste the sauce and adjust with salt and pepper as needed. Garnish with fresh parsley and serve over mashed potatoes or gluten free egg noodles.