In a small bowl, combine shrimp, the dash of salt, and the cornstarch. Divide into 4 portions; form each into a ball. Carefully drop each ball into boiling water; cook for 7 minutes, drain. Cut off and discard the base of the mushroom clusters and gently separate mushrooms. Parboil mushrooms until tender; drain. Thoroughly wash spinach; blanch, and drain. Squeeze excess liquid, cut spinach into 1 1/2-inch lengths. In a saucepan, combine dashi, sake, salt, and soy sauce; bring to a boil. To serve, place a shrimp ball, mushrooms, and spinach in each soup bowl. Add dashi and garnish with mitsuba. Makes 4 servings.