Cut stems off, leaving just ½ cm (¼ inch) attached to cap.
Put in a pot, along with enough water to cover, then add the lemon juice.
Put pot on stove, bring to a boil and simmer for 5 minutes covered.
Drain mushrooms, discard cooking liquid.
Cover drained mushrooms to keep warm, set aside.
In another pot, combine the oil, vinegar, oregano, basil and salt / salt sub.
Add onion and pimento
Bring to the boil, then remove carefully from heat.
In each quarter-litre (½ US pint) jar, put a piece of the garlic, 2 to 3 peppercorns, and optionally, a pinch of salt or salt sub and a pinch of dried chile flakes.
Fill each jar with mushrooms, leave a generous 2 cm (½ inch) headspace.
Fill each jar with oil and vinegar mixture, leaving a generous 2 cm (½ inch) headspace: stir the mixture first before adding to each jar to ensure the mixture's contents remain well-distributed.
Debubble.
Wipe jar rims well.
Put lids on.
Process in a water bath or steam canner.
Process jars for 20 minutes; increase time as needed for your altitude.