Preheat your oven to 400°F (200°C) and line a baking tray with parchment paper.
Lightly roll out the thawed puff pastry on a floured surface.
Spread a thin layer of Dijon mustard over the surface (if using).
Sprinkle evenly with chopped bacon and shredded cheddar. Season with salt and pepper.
Fold the pastry in half or roll it up tightly, then slice into 1-inch pieces.
Arrange the slices on the prepared baking tray, cut-side up.
Beat the egg and brush the tops of the pastry rounds for a golden finish.
Bake for 15–18 minutes or until golden, puffed, and crisp.
Let cool slightly before serving warm or at room temperature.