Step 1Combine the kombu and water in a saucepan and bring to a simmer over medium heat. Remove the kombu and bring the liquid to a simmer. Remove from heat. Add the shiitake mushrooms and trimmings and simmer very gently for 5 minutes. Remove from heat and strain the solids, discarding them. This makes about 1 quart dashi, more than is needed for the remainder of the recipe. The dashi will keep, covered and refrigerated, up to 5 days.
Step 1In a nonreactive pot, combine the maple syrup, vinegar, wine, dashi, rose water, peppercorns and leaf. Whisk in the salts. Bring to a boil over high heat, and then remove from heat.
Step 2Pour the hot liquid over the onions in a nonreactive container, and weight the onions down so they are completely submerged. Cover the container with cheesecloth or a towel, and keep in a cool, dry place for 1 to 2 days to quickly pickle the onions. Afterward, cover and refrigerate until ready to use.
Step 3The pickles are best if used within 1 week, but will keep, covered and refrigerated, up to 1 month.