Preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper or a silicone mat for easy cleanup.
Prepare the filling:
Heat olive oil in a pan over medium heat.
Add the chopped onion and minced garlic and sauté until softened, about 3-4 minutes.
Stir in the cooked chicken, mozzarella, and cheddar cheese.
Optionally, mix in Dijon mustard for extra flavor. Season with salt and pepper to taste. Remove from heat and set aside to cool slightly.
Roll out the puff pastry:
Unroll the puff pastry sheets onto a lightly floured surface.
If necessary, gently roll the sheets to smooth out creases or adjust the size. Cut each sheet into 2 equal rectangles.
Assemble the jalousie:
Spoon a generous amount of the chicken and cheese mixture onto the center of one rectangle of puff pastry, leaving a border around the edges.
Brush the edges of the pastry with a little water to help seal.
Place the second rectangle of puff pastry on top of the filling. Press down along the edges to seal, then crimp the edges with a fork.
Add the egg wash:
Brush the top of the assembled jalousie with the beaten egg to give it a golden, crispy finish when baked.
Bake:
Place the pastry on the prepared baking sheet and bake for 20-25 minutes, or until golden brown and puffed.
Serve:
Allow the jalousie to cool for a few minutes before slicing and serving. Garnish with fresh herbs like thyme or parsley, if desired.
Enjoy your golden, crispy chicken and cheese jalousie!