Bring water in a large saucepan to a gentle simmer. Add mushrooms and salt. Cover, and cook for 10 minutes. Drain and set aside.
In a separate large saucepan, prepare the brine by combining the 600ml water, vinegar, salt and sugar. Boil for 10 minutes.
Prepare 10 sterilised 200ml jars. In each jar layer the mushrooms, carrot, 2 onions, one bay leaf, some mustard seeds, a few allspice berries and peppercorns. Pour in the brine to cover the mushrooms.
Tighten the lids and place the jars in a large pot with water up to 3/4 their height. Bring to the boil for 20 minutes. Carefully remove the jars from pot and set aside upside down to cool.
Tip
If pearl onions are not available, try using small shallots.