In a large bowl, combine the Panko breadcrumbs and the milk. Let this panade sit for 5-10 minutes to allow the breadcrumbs to absorb the liquid.
To the panade, add the ground beef, ground pork, grated onion, beaten egg, allspice, nutmeg, 1 teaspoon salt, and 1/2 teaspoon pepper. Use your hands to gently mix until just combined. Do not overwork the mixture.
Roll the mixture into small meatballs, about 1 to 1.5 inches in diameter, yielding 24-30 meatballs. Place them on a baking sheet. For best results, chill in the refrigerator for 15-20 minutes to help them hold their shape.
In a large skillet over medium-high heat, melt 2 tablespoons of butter with 1 tablespoon of olive oil. Working in batches, brown the meatballs on all sides. They do not need to be cooked through. Transfer the browned meatballs to a clean plate and set aside. Do not wipe out the pan.
Reduce the heat to medium. Add the remaining 4 tablespoons of butter to the skillet. Once melted, whisk in the all-purpose flour and cook for 1-2 minutes, whisking constantly, to form a roux.
Slowly pour in the warm beef broth while whisking continuously to prevent lumps. Scrape the bottom of the pan to release the browned bits (fond). Bring the mixture to a simmer and cook for 3-5 minutes, until it has thickened slightly.
Whisk in the heavy cream, Worcestershire sauce, and Dijon mustard. Taste the gravy and season with additional salt and pepper as needed.
Gently return the browned meatballs and any accumulated juices to the skillet. Reduce the heat to low, cover, and let simmer for 10-15 minutes, or until the meatballs are cooked through and the gravy has thickened.