Clean the mushrooms by rubbing the dirt off with a brush or rinsing them quickly under running water. Do not soak them in water for long as they tend to become soggy. Slice it into uniform thin slices.
Cut onion and capsicum into thin slices. Heat oil in a flat pan. Choose your pan so that mushrooms should not be overcrowded. Add onion, curry leaves and salt. Saute for 2-3 minutes.
Then, add capsicum. Saute it well until onion turns light brownish color.
Crush garlic and ginger well, or make it a paste. Add ginger garlic and saute well.
Add turmeric powder, chili powder, pepper powder, coriander powder, and garam masala. Add oil as needed to fry the masala.