Add <quantity>100<unit>g</unit></quantity> of water and <quantity>100<unit>g</unit></quantity> of sugar to a saucepan

Turn the heat on to medium-low and bring it to a boil until the sugar is completely dissolved. Use a whisk to help on the process

Once it starts boiling, remove from the heat and add <quantity>100<unit>g</unit></quantity> of apple cider vinegar. Whisk to combine. The pickling brine is ready

Place <quantity>200<unit>g</unit></quantity> of beech mushrooms in a sealable jar and pour the warm pickling brine over them

Place a plastic sheet over the mushrooms to submerge them in the brine, cover with a lid, and leave the jar in a cool dark place for at least 2 days until pickled. The pickled mushrooms are ready

Add them to a sandwich or salad. Enjoy!