When you have bought more mushrooms than you are going to use up in the week ahead, a simple way to keep them is to dice and sauté them, what the French call duxelles. You can then pack the sautéed dice in a small freezer bag and dip into it whenever you want a tablespoon or so to add to a sauce, a soup, an omelet, whatever.
Cooks' Note
If you want to make up a large batch so you’ll always have this treasure at hand to give an intense mushroom flavor, just multiply the above ingredients, and be sure to use a larger pan for the increased amount.