Mushrooms can be preserved in several ways, but some species are better preserved in certain ways rather than others. What follows is a brief guide.
Freezing
Some mushrooms can be frozen raw, examples being Ceps and other boletus species as well as Hedgehog and Horn of Plenty. However, better results are usually obtained by frying in butter or oil before freezing. You can add some onion or garlic to them if you wish. This method works for any firm fleshed mushrooms, but softer species such as ink caps or Oyster mushrooms lose their texture. Mushrooms will keep like this for several months, and are ideal for adding to omelettes, casseroles or stews.
Drying
Many mushrooms can be dried and will keep very well as long as they are then kept in an airtight container; Kilner jars are best for this. Large or solid mushrooms are best sliced before drying – this will also determine whether they have been attacked by insect larvae. Smaller species such as Fairy Ring Champignons can be dried whole.
Place them on cake racks or other things that will allow air circulation under them as well as on top, until they are completely dry and crisp. They will dry in a warm room, but more quickly in an airing cupboard, on top of a radiator or boiler. Mushrooms can be dried in an oven at low temperature, (it is probably best to leave the door open, as if the temperature exceeds 60 degrees the mushrooms will blacken and the flavor will be adversely affected). Alternatively, mushrooms can be threaded onto string and dried over cookers, etc.
To rehydrate them either pour on hot or boiling water and leave for a half to one hour, or pour over cold water and leave them in the fridge overnight. The texture will not be quite as they were when fresh but is certainly good enough for soups, stews and casseroles. The soaking water makes a good stock.
Mushrooms in oil or clarified butter
Mushrooms can be preserved in sterilised jars under oil or clarified butter. If being preserved in oil, they should be sterilised first, either by briefly boiling in a mixture of vinegar and salted water or by frying and then covered with oil in the jars, making sure that no air bubbles are present. Keep the jars sealed and use quickly after opening, as they will not keep after they are exposed to the air. This method can work well, but discard any jars that show mould or bubbles developing – for this reason and because opened jars spoil quickly, several small jars are preferable to a few large ones.
Preserving mushrooms in clarified butter is very similar to the process of frying and freezing. Fry the mushrooms in butter, with any herbs or spices that you care to use, pour off any excess liquid and then pack them into jars, then cover them fully with clarified butter (also known as ghee) and seal with lids or parchment. If kept in the fridge, these will keep for several weeks.
Thanks to Wild Harvest for information