In a large pot of boiling salted water blanch spinach for 1 to 2 minutes and refresh. Squeeze water out firmly but do not wring. Lightly toss spinach to separate stems and leaves, and place in a bowl.
In a hot dry skillet, toast sesame seeds until they begin to pop. Reserve a teaspoon of toasted seeds for garnish. Transfer the rest of the seeds to a spice grinder or mortar and pestle. Grind until smooth. Pour into a bowl, if necessary, and add sugar, soy sauce and dashi. Mix well and pour over spinach. Toss to coat. Garnish with reserved seeds.