Mix soy sauce, mirin, water, sesame oil, red pepper flakes and brown sugar in a small pot over low heat. Heat, stirring, just until the sugar has fully dissolved. Whisk in the grapeseed oil and set aside to cool.
Place the cabbage, scallions, almond slivers, and uncooked noodles in a salad bowl. Pour half of the dressing over and toss until every piece of vegetable is coated. Add more dressing until the salad is well coated. The remaining dressing will keep in a sealed container in the fridge for at least 1 week.
Garnish with sesame seeds or mandarin orange slices.