This is a relatively dry curry, similar to a traditional mushroom bhaji, but with a few improvisational changes. I tend to like things on the spicy side. I usually use 3-4 hot peppers, but I only specified one below. Adjust the heat to your own tastes. You can substitute any firm, nutty tasting mushroom for the shaggy parasol.
Boil the potatoes for 5-6 minutes, or microwave, until partially cooked. Allow to cool for a few minutes then cut into cubes. Clean and chop the mushrooms. Mince the chile and garlic. Mix the yougurt and water together, stirring until creamy. Set the above ingredients aside for later. Start heating the oil on medium, and toss in the cumin and mustard seed. Cover and wait for the mustard seeds to pop. When they are done popping, add the mushrooms. Sautée the mushrooms on medium heat for about 5 minutes, until their liquid has mostly reduced. Add the minced garlic and chile, and stir. After about 5 minutes the mixture should be starting to turn slightly brown. Add the potatoes, turmeric, coriander and salt. Fry until the potatoes start to brown (about 7-10 minutes.) Now add the yougurt/water mixture and stir occasionally until most of the liquid is boiled away (about 8 minutes.) Serve with basmati rice and chutney. Serves four.