This spinach dish is standard fare for a Japanese meal. This dressing may also be used with a variety of other green vegetables, so don't hesitate to experiment. IN A SMALL SAUCEPAN, heat and ignite the sake to cook off the alcohol. Toast the sesame seeds in a ungreased skillet over medium heat for about 5 minutes or until golden brown. Stir occasionally. Put the seeds into a suribachi (Japanese mortar) and crush into a paste. (This can be ground in a food processor, however you will have to use at least twice the amount of sesame seeds.) Add the sake, sugar, soy sauce and rice vinegar and blend thoroughly. Remove from the suribachi and set aside while preparing the spinach. In a large, tied bundle, wash and rinse the spinach in cold water. Drain thoroughly. In a wide saucepan, bring about 1 cup water and 1 teaspoon salt to a boil. Cook the spinach until the leaves are barely wilted. Remove spinach and refresh with cold water. Squeeze dry of excess water. Keep the spinach in a bundle and cut off and discard the large stems. Cut the spinach into 1-inch pieces. Add the sesame dressing to the spinach and mix gently. Divide the spinach between 4 bowls and serve at room temperature or chilled. Garnish with extra sesame seeds.