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.
Add mushrooms into the pan. Sprinkle salt. Mix it with the masala. Mushrooms have a ton of water in them, which seeps out while cooking. Keep the flame in medium high heat. Don't cover the pan. Stir it every 2 minutes. Otherwise, masala might get burnt.
Make sure that water has evaporated. Taste the mushrooms and add more salt if desired. Add chopped coriander leaves. Add 2 or more teaspoons of oil and stir it continuously. I like to saute it for another 5-10 minutes.
You can make it further dry by sauteeing it for another 5 minutes or so in medium flame. Garnish with chopped coriander leaves. Enjoy with Rice, Roti or any Indian bread :)