Preheat oven to 400°F (200°C). Line a baking sheet with parchment.
Toss apple slices with lemon juice and a tiny pinch of salt to keep them from browning.
Roll pastry into a roughly 10x10-inch square and cut into 9 equal strips.
Spread a thin layer of mustard on each strip, if using, then sprinkle cheese.
Layer ham and apple slices lengthwise, overlapping slightly so edges peek out.
Starting at one end, roll each strip into a rose and place seam-side down on the sheet.
Brush with beaten egg and bake 15–18 minutes until golden and puffed. Cool 5 minutes before serving.