Clean the button mushrooms(koon) and put them for some time in medium hot water. Take out the mushrooms from water and remove water by pressing the mushrooms well. Slice the mushrooms into medium sized pieces. Keep aside.
Heat oil in a frying pan or cheenachatti and then add the chopped onion. You can also add some chopped garlic pieces along with the onion. Stir in medium flame till the onion turns golden brown in color.
Add the tomato paste and keep stirring. Cook the mix for another 5 minutes in medium flame.
Then add red chilli powder, turmeric powder, coriander powder and required quantity of salt. Saute in low flame till the raw smell goes off.
After about 2 minutes, add the mushroom pieces into this mix. Stir and mix well. Cover with a lid and cook in low flame till the mushrooms are cooked properly. Do not add water, the mushroom has to cook in its own juice.
When the mushroom pieces are cooked properly, put off the stove. Stir the mix again and add chopped coriander leaves. Tasty Mushroom Masala is ready.
If required, you can also garnish the Mushroom Masala with onion pieces. Serve with chappathi/poori/cooked rice.