In a 5- to 6-quart pan over high heat, bring about 3 quarts water to a boil. Add half the spinach and cook until wilted, 1 to 2 minutes. Remove spinach from water with a strainer and place in a large bowl of ice water until cool; lift out and drain. Repeat process to cook and cool remaining spinach.
In a large bowl, mix sake, soy sauce, sugar, and sesame oil.
Squeeze or roll spinach tightly in a kitchen towel to remove as much water as possible. Place in bowl and mix with dressing to coat. If making salad ahead, cover and chill up to 4 hours.
Divide spinach into six cylindrical bundles and squeeze each tightly to compact the leaves. Arrange bundles on a serving plate or in individual small, shallow bowls. Spoon a little of the dressing left in the bowl over each bundle. Sprinkle each bundle with about 1 teaspoon sesame seeds.