01 -
In a large bowl, combine 1/3 cup whole milk and the panko breadcrumbs. Let them sit until the milk is absorbed, about 5 minutes.
02 -
To the breadcrumb mixture, add the egg, garlic, 1 teaspoon of salt, 1/2 teaspoon of pepper, nutmeg, and allspice; mix well. Add the ground beef and ground pork; mix until thoroughly combined. Using a 1 1/2 tablespoon cookie scoop, form into about 20 meatballs.
03 -
In a large skillet, melt 1 tablespoon of butter over medium heat. Add the pierogies and 2 tablespoons of water; cover and steam until the pierogies are softened, 2 to 3 minutes. Remove the lid and allow the pierogies to pan fry until lightly golden, 1 to 2 minutes. Remove them to a plate.
04 -
In the same skillet over medium heat, add the olive oil and heat until shimmering. Add the formed meatballs in a single layer and fry, turning to cook evenly, until deeply golden and fully cooked, 7 to 9 minutes. Transfer to a plate. Pour off any excess grease.
05 -
In the same skillet, add the remaining 2 tablespoons of butter and melt over medium heat. Add the onion and cook until soft and golden, 4 to 6 minutes. Sprinkle in the flour and cook, stirring frequently, until the raw flour smell disappears, 1 to 2 minutes. Whisking constantly, slowly add the beef broth and the remaining 1/2 cup of milk. Whisk in the remaining 1/2 teaspoon of salt and 1/4 teaspoon of pepper. Simmer until the sauce thickens slightly, 2 to 3 minutes. Reduce the heat to low and whisk in the Worcestershire sauce and Dijon mustard.
06 -
Add the pierogi and meatballs to the skillet, along with any juices, stirring gently to coat them in the sauce and heat them through. To serve, sprinkle with parsley or dill, if desired.