In a mixing bowl; sift flour, add butter cubes and work it in with your fingers. Add water and kneed till dough form. Shrink wrap the dough and rest in fridge for 60 minutes.
Crush coriander and cumin seeds in a pestle & mortar.
Saute the green chilies in a pan for 3 to 4 minutes.
Add coriander, cumin and garam masala into the saute pan before adding mashed potatoes. Mix well and season with mango powder and salt. Saute for 7 to 8 minutes.
Roll out the dough to about 5mm thickness, line a drip pan with the dough, dock with a fork and trim off the extras.
At the barbecue:
Preheat grill to 190°C for 15 minutes with the lid down.
Place the dough lined drip pan over indirect heat and blind bake with the lid down for 15 minutes. Then add the filling and cover the top with the remaining rolled dough to make the top crust. Bake using indirect heat till golden brown.
Rest the pie, then serve with mint and sweet tamarind chutney.