Although it's designed to accompany Quail with Asian Spices, this is a fine stand-alone salad that is fast and easy to make, at least if you omit the crunchy leeks (which you may not want to do; they're irresistible).
Heat 2 1/2 tablespoons of vegetable oil in a large, heavy skillet. Add half the leeks, season with salt and pepper and cook over moderate heat until lightly browned, about 2 minutes; transfer to paper towels. Repeat with the remaining 2 1/2 tablespoons of vegetable oil and the remaining leeks.
In a large bowl, whisk the soy sauce with the vinegar, shallots and a large pinch of salt. Slowly whisk in the hazelnut oil. Add the watercress and cabbage to the bowl along with half the fried leeks, season with salt and pepper and toss well. Garnish with the remaining fried leeks and the chives and serve.