Prepare your jars and canning pot according to the guidelines in the Notes below.
Clean and trim mushrooms stems. If mushrooms are large halve or quarter them vertically. Button mushroom up to 1 ¼ " in diameter can stay whole.
Combine 8 cups of water with the lemon juice and bring to a boil over high heat. Add the mushrooms, reduce heat and simmer 5 minutes. Drain mushrooms in a large colander and allow them to drain and cool.
Add garlic and leaves only from 1 stem of thyme to a medium sauce pan. Add the salt, peppercorns, vinegar and remaining water to the sauce pan. Bring to a boil and let boil 1 minute. Stir to dissolve the salt. Reduce heat to low and keep brine warm. (This is about the right time to put the jar lids in boiling water.)
Take hot jars out of the oven with tongs. Set them on a dry tea towel or mat so hot jars don't sit on a cold or wet counter. Put 1 piece of thyme and rosemary in to each jar. Divide mushrooms equally between the jars. (Use a wide mouth funnel if you have one to keep jars clean.) Do not over fill the jars. You need 1" of headspace.
Spoon the garlic and peppercorns out of the brine and divide up between the jars. Ladle the hot brine over the mushrooms leaving ½" of headspace. Insert a knife in each jar to remove any trapped air.
Wipe the rim and threads of each jar with a cloth or paper towel dipped in vinegar. Remove lids from water with tongs. Center lid carefully on the jar and screw the bands on to just finger tight.
Submerge the jars in your hot canning pot and ensure water covers jars by at least 1" and preferaby 2". Cover pot and bring water to a boil. Boil for 20 minutes. Turn off the heat and let jars stay in the water for about 5 minutes more.
Place a towel on a heat proof surface. Carefully remove jars with tongs and set them on the towel. Let jars cool for 12 or so hours. Remove the screw bands and check the lids have sealed. (See Notes below.)
Store up to 1 year. Once opened refrigerate and use within a month.