Heat oven to 150°. Clean mushrooms with a brush. Try to avoid getting them wet, but if you have some stubborn dirt and must use a damp paper towel, use it sparingly. Slice them thin, about ⅛ of an inch thick. The thicker the slices, the longer they take to dry out. Arrange sliced mushrooms on baking sheets in a single layer. Cook for 1 hour.
Remove pan from oven and turn mushrooms over. Continue to cook until completely dry, at least 1 more hour. Keep turning them over and checking for dryness every 30 minutes. Cool mushrooms completely (mushrooms will crisp up as they cool) then store in an airtight container in a cool, dark place.
Tips for drying mushrooms:
• Stored properly, dried mushrooms will keep for years.
• Dried mushrooms have a more concentrated flavor than fresh.