No matter which method you choose, you'll need to clean mushrooms first. Most cooks avoid washing them, because it tends to make them mushy. Check mushrooms carefully, brushing off any visible dirt. Trim stem ends. To freeze raw mushrooms, individually quick freeze them first on a parchment-lined tray. Once mushrooms are frozen, place them into freezer bags, removing as much air as possible.
If you plan to blanch mushrooms using steam, first sort mushrooms by size. This step is necessary because blanching times vary based on mushroom size. If individual mushrooms are larger than an inch across, slice or quarter them. To keep mushrooms from darkening during blanching, soak them for five minutes in a solution of 1 teaspoon lemon juice or 1.5 teaspoons citric acid in 1 pint of water.
To blanch, add mushrooms to steamer basket over boiling water. Steam whole mushrooms 5 minutes, buttons or quarters 3.5 minutes, and slices 3 minutes. Cool mushrooms quickly by placing them in ice water for the same amount of time you steamed them. Strain, then place mushrooms in airtight freezer bags or containers.
Frozen steamed mushrooms work well in any dish, from stroganoff, to quiche, to pizza. For dishes not heading into a hot oven, like soup or quinoa, add frozen mushrooms about 20 minutes before cooking time is complete. If adding frozen mushrooms to stir fries, add just a few at a time to avoid cooling the pan. For best flavor, use frozen steamed mushrooms within a year.
Another option is to sauté mushrooms before freezing. Sauté mushrooms in a small amount of hot oil or butter. Feel free to add salt, pepper, onions, or other favorite seasonings. Sautee mushrooms about 4 minutes—until they're almost fully cooked. Allow mushrooms to cool, then pack into freezer bags or containers, removing as much air as possible. You can also individually quick freeze sautéed mushrooms on a parchment-lined tray before packing into freezer containers.
Sautéeing mushrooms before freezing produces a firmer product after freezing. Use these mushrooms in ways similar to steamed mushrooms—adding to quiche, stir fries, pizza, or pasta dishes. For best flavor, use sautéed frozen mushrooms within nine months.