Lightly wash the mince (optional) and drain on a fine slotted colander.
Heat the oil and fry the green chillies on a low heat till they turn translucent. Then add the ginger and garlic paste and saute for a few seconds. Add the chopped onions and fry till they turn translucent. Then add the tomato paste and fry for a few seconds
Reduce the heat completely and add the spice powders. Fry for a few seconds before adding the minced meat.
Fry the meat on a medium high heat till the colour deepens, around 3-4 minutes. Then add the whisked yogurt, salt to taste and chopped coriander leaves and continue to cook on medium heat till the meat is done. Add the frozen peas and continue to cook for another minute. Remove from heat and keep aside
To make the optional mushroom layer, wash and cut the mushrooms into thin slices. Heat a saucepan and add the butter & oil in it. Add the sliced mushrooms and fry them on a high heat till they soften and shrink in size. Season with salt & pepper. Remove from heat & keep aside.
Wash and pressure cook the potatoes with their skins on for about 7-8 whistles. Turn off the heat and let the pressure cooker cool down until you are able to remove the weight easily. Remove and cool them.
Skin the potatoes and mash them into a bowl. Add the egg yolk, salt, pepper & grated cheese and mash everything well again.
To assemble, grease a 9X9 inch square or 9 inch round ceramic or glass, oven safe bakeware with butter. Add the cooked mince and level off the surface. Top it up with the optional mushroom layer or directly add the mashed potato mixture
Level off the surface of the mashed potatoes, then run a fork through the surface to form a pretty design.
10. Preheat an oven at 180 degrees C and bake for 12 minutes. Then change the oven setting to 'grill' mode and grill for 3-4 minutes or till the pie gets a beautiful golden crust.
11. Remove, cool slightly and then gently run a knife through to yield 9 equal portions.
12. Serve hot immediately