Sauté Aromatics
Heat half of the olive oil in a large skillet over medium heat. Add the finely diced onion and minced garlic, stirring frequently until softened and fragrant, about 5 minutes. Allow them to cool, which integrates the flavors smoothly into the meatball mixture without cooking the meat.
Mix the Meatball Mixture
In a large bowl, combine the panko breadcrumbs, Parmesan cheese, whisked egg, milk, the cooled onion and garlic, salt, oregano, allspice, nutmeg, and pepper. Gently incorporate the ground beef and pork, mixing until evenly combined. Avoid overworking to maintain tender meatballs.
Form and Chill Meatballs
Shape the mixture into 1 ½-inch meatballs and place them on a plate. Using a small cookie scoop ensures uniform size. Chill in the refrigerator for at least 15 minutes to help them firm up, which prevents them from falling apart when cooking.
Prepare the Gravy Base
While the meatballs chill, combine the beef broth, chicken bouillon, Worcestershire sauce, Dijon mustard, and parsley in a large measuring cup. This mixture forms the flavor core of your sauce.
Brown the Meatballs
Heat the remaining olive oil over medium-high heat in a skillet. Brown the meatballs in batches, leaving space around each one. This ensures even browning and easy rotation, cooking for about one minute per side. Set aside and add oil as needed.
Make the Gravy
Melt the butter in the same skillet, using a silicone spatula to scrape up any fond for additional flavor. Stir in the flour, cooking for about 2 minutes until it becomes fragrant and begins to brown. Gradually add the beef broth mixture in splashes, stirring constantly to avoid lumps. Reduce to a simmer as it thickens.
Incorporate Sour Cream
Temper the sour cream by stirring it with a few spoons of hot sauce before adding it back to the skillet. Incorporate it over low heat to prevent curdling, maintaining smoothness in the gravy.
Combine and Serve
Return the meatballs to the skillet, including all the drippings from the plate. Spoon the sauce over them and allow to heat through, partially covered, for 10-15 minutes until cooked through. Garnish with fresh parsley and serve over mashed potatoes or egg noodles for a complete meal.