Pigs in Sleeping Bags

Name: Pigs in Sleeping Bags
Ingredients: 1 tablespoon unsalted butter; 1/4 cup minced shallot; 2 large garlic cloves, minced; 3 tablespoons panko (Japanese breadcrumbs); 3 tablespoons whole milk; 3/4 cup drained sauerkraut; 1 tablespoon Dijon mustard plus more for serving; 3/4 teaspoon caraway seeds, toasted, lightly crushed; Kosher salt and freshly ground black pepper; 8 ounces ground pork; 1 14-ounce package Dufour Pastry Kitchens frozen puff pastry, thawed; All-purpose flour (for dusting); 1 large egg, beaten to blend