Wash the mushrooms properly. (Sprinkle some All Purpose Flour/Maida on the Mushrooms generously. Now wash them under running water.)
Cut the mushrooms into slices or quarters.
Then put them directly into a pot of boiling water for 1 minute.
Wash the mushrooms in cold water to stop the cooking process and drain.
Place them in plastic freezer containers. Seal and freeze. You can store them for 2 months.
See these frozen mushrooms after a week, they still maintain their colour and texture.