How to safely home pressure can plain mushrooms, following tested USDA methods.
Pack into quarter-litre (1/2 US pint) or half-litre (US pint) jars.
To a quarter-litre (1/2 US pint) jar add 1 tablespoon cognac; double that for the half-litre (US pint) jars.
[OPTIONAL] The USDA notes: "For better color, add 1/8 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.