In a large bowl, combine whole milk with panko breadcrumbs. Let sit for about 5 minutes. Add egg, grated garlic, 1 teaspoon kosher salt, 1/2 teaspoon black pepper, nutmeg, and allspice. Mix well and add both ground beefs. Form approximately 20 meatballs using a cookie scoop.

In a large skillet, melt 1 tablespoon of salted butter over medium heat. Add frozen pierogies and 2 tablespoons of water. Cover and steam for 2-3 minutes. Remove lid and pan fry for 1-2 minutes until lightly golden. Transfer to a plate.

In the same skillet, add 2 tablespoons of olive oil over medium heat. Add meatballs in a single layer and fry for 7-9 minutes until browned and cooked through. Transfer to a plate and pour off excess grease.

In the same skillet, melt remaining 2 tablespoons of butter over medium heat. Add chopped onion and sauté until soft and golden, about 4-6 minutes. Sprinkle in flour and cook for 1-2 minutes. Gradually whisk in beef broth and remaining milk. Add 1/2 teaspoon salt and 1/4 teaspoon pepper. Simmer for 2-3 minutes until thickened.

Reduce heat to low and whisk in Worcestershire sauce and Dijon mustard. Add cooked pierogies and meatballs back into the skillet, stirring gently until well-coated with the sauce and heated through.