Clean the mushrooms by wiping with a damp cloth. Do not wash them under running water as they will absorb water and produce a diluted ketchup. Remove any obviously dirty or damaged spots. Place in a large bowl and crush them firmly with your hands until they are broken down into rough pieces.
Strow a good deal of salt generously over the crushed mushrooms. Toss to distribute the salt throughout. Cover the bowl with a cloth or plastic wrap and refrigerate for 12 to 24 hours.
Transfer the mushrooms and all their liquid to a saucepan. Add your chosen spices. Bring to a boil over medium heat and cook for 10 to 15 minutes, stirring occasionally.
Pour the cooked mushroom mixture into a cheesecloth-lined strainer set over a bowl. Gather the cheesecloth around the mushroom solids and wring firmly until every possible drop of liquid has been extracted.
Return the strained liquid to the saucepan. Bring to a simmer over medium heat and reduce until you have reached your desired consistency.
Pour the finished ketchup into a sterilised glass jar or bottle.
