While potatoes are cooking, bring water and quinoa to a boil. If using, put a piece of kombu in the pot. Cover, reduce heat and simmer for 10-20 minutes (soaked quinoa takes less time to cook than unsoaked) until all water is absorbed. Once quinoa has absorbed all the water, turn off heat and let it sit, covered, for 2-5 minutes. Then fluff with a fork. Toss or eat the kombu—it’s up to you.