Preheat your oven to 400°F (200°C).

In a large skillet, heat the olive oil over medium heat. Add the diced chicken and cook until browned on all sides, about 5-6 minutes. Remove chicken from the pan and set aside.

In the same skillet, melt the butter. Add the sliced leeks and cook until softened, about 5 minutes.

Sprinkle the flour over the leeks and stir constantly for 1 minute to cook out the raw flour taste.

Gradually pour in the chicken broth while stirring to prevent lumps. Simmer for 2-3 minutes until the sauce thickens.

Stir in the heavy cream, Dijon mustard, and dried thyme. Return the cooked chicken to the pan and add the cubed ham. Stir well to coat everything in the sauce. Season with salt and pepper to taste.

Transfer the filling mixture into a deep 9-inch pie dish or baking dish.

Roll out the puff pastry sheet slightly to fit over your dish. Place the pastry over the filling, crimping the edges against the dish to seal. Cut a few slits in the top to allow steam to escape.

Brush the top of the pastry with the beaten egg.

Bake for 25-30 minutes, or until the pastry is puffed and golden brown and the filling is bubbling hot.