If starting with raw sesame seeds, add them to a dry pan over medium heat. Toast for 3-5 minutes, shaking or stirring often, until lightly golden and fragrant.
Transfer the warm sesame seeds to a mortar and pestle. Crush them until mostly broken down, leaving some texture.
In a bowl, add the crushed sesame seeds. Mix in the Kewpie mayo, rice vinegar, white sugar, soy sauce, water, and sesame oil.
Mix til well combined. The dressing should be creamy with little bits of toasted sesame throughout.
Taste the dressing and feel free to adjust with more rice vinegar for acidity, sugar for sweetness, soy sauce for saltiness, or sesame oil for extra aroma.
Add more water, 1 tsp at a time, to loosen the dressing as needed. Keep it thicker for hearty salads or add a bit more water if you want it easier to drizzle.
Use the dressing right away or chill until ready to serve.
Spoon or drizzle over warm mushroom salad, greens, tofu bowls, cold noodles, roasted vegetables, or anything of your choice.
