Start by making the dressing for your coleslaw. Take a small clean bowl and put 5 teaspoons of wasabi powder in it. Add 2 1/2 teaspoons of water to the bowl and mix well until a smooth paste forms. Set aside.
Take another non-reactive bowl, such as a glass bowl, and pour two tablespoons of rice wine vinegar or apple cider vinegar into it. If using cider vinegar, add two teaspoons of sugar and whisk until the sugar is dissolved.
Pour 1/3 cup of mayonnaise into the sugar and vinegar syrup and whisk to combine.
Now, add the wasabi paste to the bowl and mix well. Also add ¼ cup grape seed oil to the dressing.
Season your dressing with salt and pepper to taste, and allow it to stand in the refrigerator for 10 to 15 minutes.
Meanwhile, wash the vegetables and set them aside to dry completely.
Using a sharp knife thinly slice the cabbage and green onions, and set aside.
Finely shred the carrot and red cabbage, and mince the cilantro, if using.
Put all the vegetables in a large non-reactive bowl, mix well, and set aside.
Just before serving the salad, remove the wasabi dressing from the fridge and add it to the bowl containing the vegetables.
Stir the vegetables well to evenly combine them with the dressing.
Your wasabi coleslaw is now ready. Garnish it with a sprinkling of black sesame seeds, and serve cold.