Preheat your oven to 400°F (200°C).

In a large skillet or pot, melt 1 tablespoon of butter over medium-high heat. Add the chicken chunks, season with salt and pepper, and cook until browned on all sides (about 5-6 minutes). Remove chicken and set aside.

Reduce heat to medium. Add the remaining 2 tablespoons of butter to the pan. Add the sliced leeks and cook for 5 minutes until softened and fragrant.

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

Gradually pour in the cider (or broth) while whisking to prevent lumps. Simmer for 2-3 minutes until the liquid reduces slightly.

Stir in the heavy cream, whole grain mustard, and dried thyme. Simmer for another 2 minutes until the sauce thickens.

Return the cooked chicken and the cubed ham to the pan. Stir to coat everything in the sauce. Taste and adjust salt and pepper if needed.

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

Roll out the thawed puff pastry slightly to fit over your dish. Place the pastry over the filling, crimping the edges against the rim of the dish to seal.

Cut a few small slits in the center of the pastry to allow steam to escape. Brush the top generously with the beaten egg.

Bake for 20-25 minutes, or until the pastry is puffed and deep golden brown. Let rest for 5 minutes before serving.