This English condiment is thin, pungent, and deeply flavored. Please read about pickling and How to Process.
Wash and coarsely chop:
Spread the mushrooms out in a large baking dish and sprinkle with:
Cover and refrigerate, stirring occasionally, for 2 days.
Drain the mushrooms and rinse well. Combine in a large saucepan with:
Bring to a boil, then reduce the heat and simmer uncovered, stirring often, until very fragrant, flavorful, and reduced by half, about 30 minutes.
Prepare a water-bath canner, gather all your canning equipment, and have ready 3 clean, hot half-pint jars. Strain through a fine-mesh sieve, then again through a sieve lined with several layers of cheesecloth or a thin kitchen towel. Pour the hot catsup into the prepared jars, leaving ½ inch headspace. Wipe the rims. Place lids on jars and screw on rings until fingertip-tight. Process for 15 minutes. Let cool completely and store as directed.