Start by cleaning the mushrooms thoroughly, then slice them into small cubes. Bring a large pot of water to a boil, and drop the mushrooms in. Let them simmer for about 20 minutes . This removes oxalic acid and makes the mushrooms tender and safe to preserve.
While the mushrooms are bubbling away, set up a second pot. Combine sugar and water in it, then place it over low heat. Allow the syrup to warm slowly until it’s clear and smooth, keeping it on a gentle simmer as you finish preparing the mushrooms.
While both pots are cooking, get the rest of your ingredients ready. Grate the lemon to collect the zest, then cut it into thin slices. Slice the orange as well, and measure out the cinnamon, rosemary, and cloves.
After the mushrooms have boiled for 20 minutes, strain them well and transfer them straight into the pot with the syrup. Stir in the citrus slices, lemon zest, cinnamon, rosemary, and cloves. Keep the heat low and let the mixture cook slowly until the syrup thickens and turns glossy.
While the preserve is simmering, sterilize your jars and lids. You can do this by boiling them in water or heating them in the oven at 212°F/100°C . Keep the jars warm until you’re ready to fill them and seal tightly once filled with hot syrup.
To test whether the preserve is ready, do a quick plate test. Drop a bit of the syrup onto a cold plate - if it forms a thick, sticky line that doesn’t run, you’re good to go. If it still spreads easily, cook it for a few more minutes and test again.
Once the preserve reaches the right consistency, carefully ladle it into the hot, sterilized jars. Seal each one tightly, then turn the jars upside down and wrap them in a thick towel or blanket. Let them cool slowly overnight.
If you’d like to enjoy some right away, resist the urge to eat it warm. Let it cool completely, preferably overnight. By morning, the mushrooms will have soaked up all the syrup and turned wonderfully soft, bright, and full of flavor. Enjoy!