Preparation:

1. Preheat the oven to 200°C (390°F).

Making the Filling:

1. Brown the Chicken: In a large pan, heat the vegetable oil over medium-high heat. Add the chicken pieces and cook until browned. Remove the chicken and set aside.
2. Cook the Aromatics: In the same pan, add the onions and cook until soft. Then add the garlic, ginger, and brown mustard seeds, cooking until the seeds start to pop.
3. Add the Spices: Stir in the ground cumin, coriander, turmeric, and chili powder, cooking for a minute until fragrant.
4. Combine: Return the chicken to the pan, add the chopped tomatoes, and season with salt. Simmer for about 15 minutes until the chicken is cooked through and the sauce has thickened. Stir in the garam masala.
5. Cool: Allow the filling to cool slightly before assembling the pie. This prevents the pastry from becoming soggy.

Assembling the Pie:

1. Prepare the Pastry: Unroll the puff pastry. Cut a piece large enough to cover your pie dish with some overhang.
2. Fill the Pie: Spoon the chicken balti filling into a pie dish. Place the pastry over the top, pressing down the edges to seal. Trim any excess pastry.
3. Finish: Brush the top of the pie with the beaten egg for a golden finish.
4. Bake: Bake in the preheated oven for 25-30 minutes, or until the pastry is puffed and golden brown.

Serving:

1. Rest: Let the pie sit for a few minutes before serving.
2. Garnish and Serve: Garnish with fresh coriander. Serve hot with your choice of side.