If you don’t have shiitake mushrooms, use baby bella (cremini). Or use a mix!
To make quick pickled red onions, pour about 1/2 cup (120 ml) pickling liquid from your cucumber pickles into a small container. Thinly slice 1/4 of red onion, add to the pickling liquid and let stand at room temperature for a few hours. Keeps chilled for up to a week!