In a large mixing bowl, combine the Panko breadcrumbs and milk. Let it sit for 5-10 minutes until the breadcrumbs have absorbed the liquid.

Add the ground beef, ground pork, lightly beaten egg, finely minced onion, minced garlic, Worcestershire sauce, dried parsley, salt, and pepper to the bowl with the soaked breadcrumbs.

Using your hands, gently mix all the ingredients until just combined. Be careful not to overmix, as this can make the meatballs tough. Form the mixture into 1.5-inch balls (you should get about 24-28 meatballs).

Heat the olive oil in a large skillet or Dutch oven over medium-high heat. Working in batches to avoid overcrowding the pan, sear the meatballs on all sides until nicely browned. This should take about 5-7 minutes per batch. The meatballs don't need to be cooked through at this stage. Transfer the seared meatballs to a plate and set aside.

Reduce the heat to medium. Add the butter to the same skillet, scraping up any browned bits (fond) from the bottom of the pan. Add the chopped onion and sauté for 4-5 minutes until softened and translucent.

Sprinkle the flour over the onions and cook, stirring constantly with a whisk, for about 1-2 minutes to cook out the raw flour taste. This is your roux.

Gradually pour in the beef broth while whisking continuously to prevent lumps. Whisk until the gravy is smooth. Add the Worcestershire sauce and Dijon mustard, and stir to combine.

Bring the gravy to a simmer and let it cook for 5-7 minutes, stirring occasionally, until it has thickened enough to coat the back of a spoon.

Gently return the seared meatballs (and any juices from the plate) to the skillet with the gravy. Reduce the heat to low, cover, and let simmer for 15-20 minutes, or until the meatballs are cooked through and tender.

Remove the skillet from the heat and slowly stir in the heavy cream or sour cream until well combined. Season with additional salt and pepper to taste. Garnish with fresh parsley before serving.