Combine all ingredients except the mushrooms in a bowl and warm au bain marie for 20 minutes. Remove the bowl from the heat, let cool and infuse overnight at room temperature.
Strain the cooled pickling liquid the following day and mix with the mushrooms. Allow to macerate for 30 min, mixing occasionally. Then make sure the mushrooms are submerged under the pickle liquid and refrigerate. They will keep for approximately 1 month.