Heat oil in a large kadhai or wok. Add mustard seeds, ginger, garlic, curry leaves, dried red chillies and stir fry for two minutes till you can start smelling all the aromatics.
Add onions and cook till translucent. Add tomato paste, coriander powder, cumin powder, chilli powder and garam masala and cook the tomato paste for 2-3 minutes on high heat till you can see oil droplets around the sides.
Add the mushrooms, pepper and salt and cook on high heat, tossing regularly for a few minutes till the mushrooms leave water and reduce slightly in size. Cooking on high heat prevents them from becoming soggy. Once the mushrooms are cooked, turn off the heat and serve.
Use fresh mushrooms, I used buttonmushrooms. Make sure you thoroughly clean the mushrooms. If necessary, soak the mushrooms in water along with 1-2 teaspoons of vinegar. Rinse, cut and use.
Do not cut the mushrooms well in advance as they will start to lose moisture. Cut and use immediately. If the mushrooms are very big, then cut it in quarters. If it is very small, use them whole.
Use freshly ground black pepper in place of store bought pepper powder. You can also dry roast the black pepper for 1-2 minutes before grinding, this will give it an extra kick.
Prepare this dish ready to serve as mushrooms will start to leave water when stored for long time. It tastes best when freshly made and the mushrooms still have a certain crunch.