Cook the chicken in a pressure cooker with the turmeric and red chilli powders along with some salt till done. Reserve the stock aside - I got around 100 ml.
Preheat the oven to 180 degrees.
Heat butter in a pan. Saute the garlic till a nice aroma comes.
Add the onions and saute till transparent. Add the flour and saute for a couple of minutes.
Add the milk and whisk till there are no lumps.
Cook till the mixture starts thickening.
Add the chicken stock and mix well.
Add the frozen vegetables and cook for around 5 minutes.
Add the cooked chicken, adjust the salt and pepper.
Finally add the garam masala powder and coriander leaves and cook for a further 2 minutes.
Switch off and transfer this chicken mixture to a baking tray.
Line the puff pastry squares on the top. Brush with the egg on top.
Bake for 30 minutes till the top is browned. Serve warm!
Notes
Instead of using cooking the chicken, you can use leftover chicken and make it even more easier. The spice levels were very mild. You may add chopped green chillies for more spice.