Wash your jars and keep them warm.
Add rings and lids to a pot, cover with water and bring to a boil.
Fill your canner with warm water according to the directions.
Fill a stockpot with the water, vinegar, salt and bring to a boil.
Add the garlic, thyme, and bay leaves.
Boil for 5 minutes.
Remove the thyme and place one or two sprigs in each jar.
Remove the bay leaves and discard.
Add the mushrooms, and return to a simmer for 5 more minutes.
Pack each jar with mushrooms and 1 or 2 cloves of garlic. Leave a little over ½” headspace.
Add hot brine to each jar, maintaining ½” headspace.
Clean off the rim with a paper towel dampened with vinegar.
Add lids and rings. Center the lids on the jars. Finger tighten the rings - don’t crank down on them.
Add jars to the canner.
Make sure the jars are covered with about 1” of water (add more hot water if necessary).
Turn the heat on high.
Bring the canner to a boil.
Process the jars for 15 minutes.
Turn off the heat and remove the canner lid.
Remove the jars.
Allow the jars to cool and come to room temperature. Listen for the “ping” of the lids sealing.
Test the lids to make sure they have dimpled and no longer flex.
Once cooled, remove the rings and give the jars a quick rinse.
Label and date each jar.