Begin by warming half of the olive oil in a deep skillet over medium heat. Sauté the diced onions and minced garlic until soft and fragrant, about 5 minutes. Remove from heat and let the mixture cool completely to avoid scrambling the egg later.

In a large mixing bowl, combine the panko breadcrumbs, Parmesan, egg, milk, and the cooled onion-garlic mixture. Add the salt, pepper, oregano, allspice, and nutmeg. Gently fold in the ground beef, mixing just until everything is uniformly combined—be careful not to overmix to maintain a tender texture.

Using clean hands or a small cookie scoop, shape the mixture into 1½-inch meatballs and arrange them on a plate. Refrigerate for at least 15 minutes to help them firm up, or cover and chill overnight if preparing in advance.

Meanwhile, in a measuring cup or bowl with a spout, whisk together the beef broth, chicken bouillon, Worcestershire sauce, Dijon mustard, and dried parsley. Set this aside for the sauce.

Heat the remaining olive oil in a large skillet over medium-high heat. Brown the meatballs in batches to avoid crowding—leave space between each to turn easily. Sear each side for about 1 minute or until evenly golden. Transfer them to a plate once browned. Add more oil or reduce the heat slightly between batches if necessary.

In the same skillet, melt the butter over medium heat. Use a silicone spatula to scrape up any browned bits from the bottom—this adds flavor to the sauce. Sprinkle in the flour and stir continuously for 2 minutes, allowing it to cook and take on a light golden hue.

Gradually pour in the broth mixture in small increments, stirring constantly to create a smooth base. Bring the sauce to a gentle boil, then reduce the heat to a simmer.

Place the sour cream in a separate bowl. To temper it, add a small amount of the warm sauce (about ¼ cup) and stir until smooth. Slowly incorporate the tempered sour cream back into the skillet, stirring over low heat until fully combined and creamy.

Return the meatballs to the pan along with any juices that have collected on the plate. Gently spoon the sauce over them, cover the pan partially, and allow everything to simmer over low heat for 10 to 15 minutes, or until the meatballs are cooked through.

Finish with a sprinkle of fresh parsley and serve warm over a bed of mashed potatoes or buttered egg noodles.