Make the Dough:
In a large bowl, whisk together flour, salt, sugar, and baking powder. Add cold, cubed butter and cut it in using a pastry cutter or fingers until the mixture resembles coarse crumbs. Stir in milk and 1 egg until just combined. Do not overmix.

Chill the Dough:
Turn dough onto a floured surface, knead gently 2–3 times, shape into a disk, wrap in plastic, and refrigerate for at least 30 minutes.

Prepare the Filling:
While the dough chills, scramble eggs and cook sausage (or veggies/bacon) in a skillet. Let cool slightly. In a bowl, mix cooked eggs, sausage, shredded cheese, and Dijon mustard (if using).

Assemble the Pockets:
Roll chilled dough to ¼-inch thickness and cut into 4×6-inch rectangles. Preheat oven to 400°F (200°C) and line a baking sheet with parchment. Spoon filling into the center of each rectangle. Fold dough over, seal edges, and crimp with a fork.

Bake:
Place pockets on the prepared baking sheet. Brush tops with beaten egg. Bake for 20–25 minutes, or until golden and puffed. Cool slightly before serving.