Mushrooms are one of those frustrating ingredients that are really useful, but only last a few days in the fridge. If you have the opportunity to stock up on them in a big way and don't have a dehydrator, freezing them is the best way to preserve them for later use.
Freezing Mushrooms Isn't Complicated
Mushrooms aren't much like other vegetables – the question of whether they're a plant at all is hotly debated in scientific circles – but when you freeze them, you treat them in much the same way as a vegetable.
If you wanted to, you could just throw your mushrooms into the freezer as-is in a freezer bag. They'd stay food safe indefinitely, but they wouldn't be very satisfactory. White button mushrooms have a high moisture content, and if they were frozen whole, they'd be soggy when they're thawed. They'd also darken dramatically, so between their appearance and their texture, they'd be pretty nasty.
Blanching and Freezing
Mushrooms contain enzymes that try to break them down into compost once they're picked. That's great in nature, but not so good in your freezer. Heating the mushrooms stops those enzymes from working. You can do that by cooking them or by just blanching them.
"Blanching" just means heating the mushrooms briefly in boiling water or over steam. Either method works, though steam gives the best results. Slice or quarter the mushrooms or leave really small ones whole. They'll keep their color better if you soak them in a mixture of water and lemon juice for a few minutes before blanching.
Steam the mushrooms for 3 to 7 minutes, depending on how large you've cut them; then transfer them to a baking sheet or shallow tray to cool. You can freeze them on the sheet in a single layer and then bag them after they're frozen, or bag them first and then freeze. Freezing them separately means you can open the bag and take out as many as you want at a time. If you bag them first, you'll need to thaw the whole bag, so be sure to portion them in useful quantities.
Sauteing Your Mushrooms
Freezing mushrooms after blanching does little for their flavor. Some cooks prefer to saute the mushrooms before freezing, which gives the opportunity to add flavor though your choice of cooking fat – butter is especially good for that – or with added onion, garlic, herbs or spices.
Clean the mushrooms and slice them; then saute them in a medium-hot pan until most of their moisture has been released and cooked down. It's best to do this in small batches rather than crowding the pan with a large batch. Once the mushrooms are cooked and cooled, you'll have the same option of packing them into bags immediately or freezing them first in a single layer.
Sauteed mushrooms won't last as long in the freezer, because the fat you've added will begin to develop "off" flavors after a couple of months. The trade-off is that their flavor and texture are better, and because they're fully cooked, you can add them directly to nearly-finished dishes.
Using Your Frozen Mushrooms
Freezing mushrooms keeps them usable for longer, but it does cut down on the ways you can utilize them. Thawed mushrooms won't work very well in a salad, for example, because of the change in their texture. They're best used in soups, sauces, stir-fries and similar dishes in which their excess moisture can be cooked away or concealed in the finished dish.
Mushrooms you've sauteed before freezing are more versatile, and they work well in omelets, quiches and similar dishes. You can even use them on your steak or burger. Their texture will be slightly different, but it won't be noticeable at the table.
Duxelles, a Special Case
As with any other food, you'll ideally pick only the freshest, most perfect mushrooms to prepare and freeze. So what do you do with the other ones, those that are less than perfect? One versatile option is a mushroom preparation the French call "duxelles." It's a sort of flavorful mushroom paste, which takes only a few minutes to make up and has lots of culinary uses.
Basically, all you're doing is chopping the mushrooms finely or pulsing them in a food processor, then cooking them in a skillet with butter, garlic, herbs, finely diced onion – the French use shallots – and a splash of dry sherry or dry white wine. Cook and stir the mixture until it cooks down to a paste; then portion the paste and freeze it.
You can use duxelles to stuff chicken breasts or thick pork chops, as the filling for mushroom ravioli, or as an appetizer in pastry crust or on toasted baguette slices. It's also great in any sauce or rice dish for a quick hit of mushroom flavor.