In a large mixing bowl, whisk together the all-purpose flour, salt, sugar, and baking powder until well combined. Cut in the cold butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs.

Combine the milk and egg in a separate bowl, then pour into the dry ingredients and mix until a soft dough forms, about 1-2 minutes.

On a lightly floured surface, gently knead the dough just until smooth. Roll out the dough to about 1/4 inch thick and cut it into 6-8 equal rectangles.

In a medium bowl, combine the cooked sausage, scrambled eggs, shredded cheese, and optional Dijon mustard until all ingredients are evenly coated.

Place a generous spoonful of the filling on one half of each rectangle, fold over to create a pocket, and crimp edges with a fork.

Arrange filled pockets on a parchment-lined baking sheet, brush tops with a beaten egg, and bake at 375°F (190°C) for 20-25 minutes until golden brown.

Let cool slightly on the baking sheet for about 5 minutes before serving warm.