Hearty Pierogi and Swedish-Style Meatball Skillet Dinner

Ingredients

• 1/3 cup whole milk
• 2/3 cup panko breadcrumbs
• 1 large egg
• 2 garlic cloves, grated
• 1 1/2 teaspoons kosher salt, divided
• 3/4 teaspoon ground black pepper, divided
• 1/4 teaspoon ground nutmeg
• 1/4 teaspoon ground allspice
• 1/2 pound lean ground beef
• 1/2 pound ground pork
• 3 tablespoons salted butter, divided
• 1 box (16 oz) frozen pierogies
• 2 tablespoons olive oil
• 1/2 yellow onion, chopped
• 2 tablespoons all-purpose flour
• 1 1/2 cups beef broth
• 1 teaspoon Worcestershire sauce
• 1 teaspoon Dijon mustard
• 2 tablespoons chopped fresh parsley or dill, for serving