Get a clean jar ready. If you're planning to keep these in the fridge for more than a day or two, then I'd recommend sterilising the jar (the easiest way to do that is in the dishwasher, or you can boil the jar in a saucepan for a few minutes).
Prepare your mushrooms. If they're really small you don't need to do anything, just make sure they're perfectly clean. If they're a little bigger then cut them in half or into quarters. Remove the stems, if you like.
Put the mushrooms into the jar.
Put all remaining ingredients into a pot and bring to a simmer. Turn off the heat and pour the mixture over the mushrooms (while still hot). Give everything a shake or stir, and leave to cool to room temperature. Once cool, store in the fridge.
Makes approximately 2 cups.