Preheat your oven to 400°F (200°C).
Line your baking sheet with parchment paper.
This prevents sticking and ensures easy cleanup.
Unroll the puff pastry on a clean surface.
Let it sit for a few minutes.
Spread Dijon mustard down the center of the pastry.
Leave about 3 inches in the middle.
Sprinkle the grated Gruyère cheese over the mustard.
Add thyme, salt, and pepper on top.
Don't overfill or it will leak out.
Cut diagonal strips on both sides of the filling.
Make them about 1 inch wide.
Leave the center untouched where the cheese sits.
Fold the strips over the filling alternately.
Create a braided pattern by crisscrossing each strip.
Tuck the ends underneath to seal.
Brush the beaten egg over the entire braid.
This creates that beautiful golden color.
Place the baking sheet in the preheated oven.
Bake for 20-25 minutes until golden brown.
The pastry should be puffed and crispy.
Remove from the oven and let cool slightly.
Wait about 5 minutes before slicing.
Cut into thick slices and serve warm.
Enjoy the melted cheese and flaky pastry!