Rinse hiziki under water and soak for 10 minutes. Be sure that you get all the gritty material out of the hiziki, and drain. Place about 1/2 cup of water in a skillet and bring to a boil. Cut onion and carrot into thin slices and poach in the water for a few minutes. Place hiziki on top of vegetables and add enough water to cover the vegetables but not the hiziki. Simmer 40 to 50 minutes, adding water if necessary. Season with tamari for a mild taste and continue to simmer until excess liquid is boiled off. Garnish with scallion and serve. For variety, use parsnips instead of carrot, onion and lotus root, or shiitake mushrooms and diced fresh tofu.