In a large bowl, combine 2 cups all-purpose flour and 1/2 tsp salt. Add 3/4 cup cold unsalted butter cut into small cubes. Using a pastry cutter or your fingers, work the butter into the flour until mixture resembles coarse crumbs with some pea-sized pieces remaining.

Add water

Sprinkle 4 tablespoons ice water over the flour-butter mixture, 1 tablespoon at a time, tossing gently with a fork after each addition until the dough just comes together when pressed. Do not overwork; dough should be slightly shaggy but cohesive.

Chill

Form dough into a disk, wrap tightly in plastic wrap, and refrigerate for at least 30 minutes (up to 2 hours). This chills the butter and relaxes the gluten for a flakier pastry.

Roll out

On a lightly floured surface, roll the chilled dough to roughly a 12x10-inch rectangle about 1/8–1/4 inch thick. If dough resists, return to fridge for 10 minutes and retry.

Filling

Prepare filling

Preheat oven to 400°F (200°C). In a small bowl combine 2 tablespoons Dijon mustard and 1/4 tsp black pepper. Thinly slice 2 green onions. Grate 1.5 cups sharp cheddar cheese.

Assemble layers

Spread the Dijon mustard mixture evenly over the rolled pastry, leaving a 1/2-inch border on all sides. Layer 8 slices of deli ham over the mustard in an even layer, then sprinkle the grated cheddar cheese evenly on top. Scatter the sliced green onions over the cheese.

Fold or roll

There are two serving options: For a folded turnover-style pocket, fold the pastry over the filling to create a long log and seal edges by pressing with a fork, trimming excess dough; cut into 6 equal pieces. For a single-sheet bake (preferred for even bake and presentation), roll the filled rectangle gently from one long side to the other to form a loose log, then pinch seam and ends to seal and place seam-side down on a parchment-lined baking sheet. Alternatively, simply leave as an open-faced rectangle and fold the 1/2-inch border inward to create a rim.

Finish / Bake

Egg wash

In a small bowl, whisk together 1 large egg and 1 tablespoon milk. Brush the egg wash evenly over the pastry surface and edges. If desired, sprinkle 1 tsp sesame seeds over the top.

Bake

Place the prepared pastry on a baking sheet and bake in preheated 400°F (200°C) oven for 20–25 minutes, or until pastry is golden brown and cheese is bubbling. If baking pieces, check at 15 minutes and rotate pan for even browning.

Rest and serve

Remove from oven and let rest 5 minutes on the baking sheet. Transfer to a wire rack to cool slightly, then slice if necessary and serve warm.