If you have a food processor you can use the slicer plate and thin slice any that are up to about 1.5 inches across .
Oven dehydrate them in a low oven say 50 - 60 oC with the door slightly ajar putting them on some of these pizza baking mesh mats or bamboo table place mats.
Starting the session at say 21.00 hrs turning them over hourly , doing it over night , turning them just before bed time and upon getting up .
When they are dry & leathery after around 15 hrs of drying , pop them in clean airtight jars or vac pack & heat seal them , then pop them in the freezer in small packs where they will stay OK for many months .
Another thing you can do is to make a mushroom sauce/gravy and freeze it in ice cubes
Simply cut up half a pound or so of mushrooms fry them in a big frying pan with a little butter . Once they are all buttered , put the lid on the pan, turn the heat down and let them sweat . Now pour 3/4 of a pint of boiling water in the frying pan and replace the lid let them simmer for 15 min or so . drain the elixir off using a sieve and when it is cool pop it in the cube trays and freeze .
Once frozen solid , eject the cubes and wrap them in several turns of cling film to make them air tight . Store them in a zip-lock type bag in the freezer.
Taking out a cube or two when needed for use in soups , stews or pasta dishes as needed . Don't put salt, herbs or spices in the elixir before freezing for salt will stop it from freezing and the other stuff will lose its flavour very quickly , only season the dish at the time of making the meal .
You can use this elixir for flavouring home made pate as well .