Clean the mushrooms,pull them apart into individual stems and caps. Chop the mushrooms into small chunks.
Combine the vinegar, water, salt and sugar in a small pot. Bring this to a boil and then throw in the mushrooms. Stir the mushrooms around the simmering brine until they have softened. That should take two to four minutes. Scoop the mushrooms out with a slotted spoon or a fork onto a paper-towel-lined tray. Cover the mushrooms with another paper towel and press lightly to absorb excess liquid. Let these air dry while you prepare your jar.
Sterilize one eight-ounce Mason jar, ring and lid. Next, fill a crock-pot/slow cooker with water and set to low.
In the sterile jar, place the bay leaf, thyme, juniper and pickled mushrooms.
Cover the mushrooms with oil and use a chopstick to poke around the jar, releasing any trapped air bubbles.
Once everything is well covered with oil, put the lid on the jar and place inside the crock-pot for 90 minutes to two hours.
After cooking, remove and cool before placing in your fridge. This will keep in the fridge for at least a month if kept sealed. Serve with bread and sliced brie.