In a blender blend mustard seeds, garlic cloves and green chiilli with very little water to form a fine paste.
Heat oil in a wok. Add the above paste and saute for two minutes. Add the fried potatoes and saute well.
Add tomatoes and salt. Cover and cook for five minutes.
Add the fried mushrooms and toss well. Add ¼ cup water and mix.
Cover and cook for five minutes.