Pound chiles, garlic, 1/2 tsp. sugar, and salt into paste in a mortar, or in a blender. Transfer paste to a small bowl and add remaining 1 tsp. sugar, the fish sauce, and rice vinegar.
Combine carrot, fennel, cabbage, and mint in a large bowl. Pour dressing over the vegetables, tossing to coat evenly. Sprinkle with cashews just before serving.
Make ahead: Up to 2 days, slaw mixture and dressing. Just before serving, combine with nuts.