Trim off and discard stem ends and any discoloured parts.
Soak in a sink, or large pot or bowl, full of cold water. Soak for 10 minutes to loosen any dirt.
Refill sink with fresh clean cold water, and wash.
You can leave small mushrooms whole; cut large ones up.
Put mushrooms in a pot with water, bring to a boil and boil 5 minutes.
Pack into quarter-litre (½ US pint) or half-litre (US pint) jars.
To a quarter-litre (½ US pint) jar add 1 tablespoon cognac; double that for the half-litre (US pint) jars.
Leave 3 cm (1 inch) headspace.
Top up with clean boiling water (such as from a kettle, for instance), maintaining headspace.
Debubble, adjust headspace.
Wipe jar rims.
Put lids on.
Processing pressure: 10 lbs (69 kPa) weighted gauge, 11 lbs (76 kpa) dial gauge (adjust pressure for your altitude when over 300 metres / 1000 feet)
Processing time: Either size jar for 45 minutes.
Notes
[OPTIONAL] The USDA notes: "For better color, add ⅛ teaspoon of ascorbic acid powder, or a 500-milligram tablet of vitamin C."Instead of cognac, you could try an equivalent amount of brandy, or red or white vermouth.