1. In a small mixing bowl, combine the rice vinegar, soy sauce, and bonito flakes. Let the ingredients infuse at least 4 hours or overnight. Strain through a cheesecloth.

2. Bring a large pot of salted water to boil. Add the spinach, and cook for 2 minutes. Plunge the spinach into a bowl of ice water to stop the cooking, and drain in a colander. Using your hands, squeeze out any excess water from the spinach.

3. Divide the spinach into four portions. Using a bamboo sushi roller, roll each portion of the spinach into a tight cylinder, four inches long. Slice each cylinder across the middle on an angle.

4. Place two spinach rolls on each plate; drizzle 2 tablespoons of sauce over each serving.