Place all of the ingredients except the mushrooms into a saucepan and add 500ml/18fl oz water.
Bring to the boil and add the mushrooms. Cook for 12-15 minutes, or until cooked through.
Remove the mushrooms from the pan with a slotted spoon and set aside to dry on a clean tea towel for 3-4 hours, or until completely dry.
Pour some olive oil into a 1 litre/1¾ pints sterilised jar, add the mushrooms, then cover with more olive oil. Stir together gently until all of the mushrooms are coated in the olive oil. Cover tightly with a sterilised lid.
The mushrooms will keep for two months unopened. Use within 3-4 days once opened.