Although possibly the best way to dry mushrooms is in a dehydrator, you can get very good results using your home oven.
This is a great way to preserve an abundance of foraged mushrooms, or simply to make sure that the store-bought mushrooms in your refrigerator don't spoil before you get around to eating them.
Most mushrooms dry beautifully, not only retaining but even gaining flavor in the process. When rehydrated in hot water, their texture is almost identical to fresh mushrooms. Dried mushrooms can be stored for at least a year.
Preheat the oven to 150F/65C.
First, clean the mushrooms. Contrary to culinary myth, fresh mushrooms do not actually soak up lots of water, and you don't have to skip washing them and get all of the dirt off with a brush.
Go ahead and wash them in water, scrubbing gently with a vegetable or mushroom brush as necessary. This will make your prep much easier, especially with species that have lots of nooks and crannies, such as maitake.
Depending on the shape of the mushroom species, it may be easiest to leave them whole when you clean them, or to slice first and then wash.
Slice the mushrooms 1/8 to 1/4-inch thick. The thicker the slices, the longer they will take to dry.
Spread the sliced mushrooms on a clean dishtowel. Loosely roll up the towel and squeeze gently to remove any surface moisture.
Arrange the cleaned, sliced mushrooms on the baking sheet, making sure that none of the pieces touch or overlap.
Place the baking sheet in the oven and leave for 1 hour.
Turn the mushroom slices over and return to the oven for another hour. Take them out of the oven and let them cool before checking to see if they are crispy-dry. The cooling off period is essential because, like cookies coming out of the oven, they will crisp up considerably as they cool.
If the mushrooms are not completely dry, turn them over again and return them to the oven for 30 more minutes. Keep turning them over and checking for dryness every 1/2 hour.
Allow the dried mushrooms to cool completely before transferring them to glass jars. Cover tightly with lids and store away from direct light or heat.
To use dehydrated mushrooms, pour boiling water over them in a heat-proof bowl. Let them soak for 20 to 30 minutes. Drain, reserving the richly flavored soaking liquid for soup stocks and sauces. Use the rehydrated mushrooms as you would fresh mushrooms.