Preheat oven to 450°. Place cabbage and onions in a large bowl. Drizzle with oil; sprinkle with salt and pepper and toss to coat. Transfer to a shallow roasting pan, spreading evenly. Roast until vegetables are tender and lightly browned, 30-35 minutes, stirring halfway., Transfer cabbage mixture to a large bowl. Add chives and tarragon; toss to combine. In a small bowl, whisk dressing ingredients until blended. Drizzle over cabbage mixture; toss to coat. Let stand 10 minutes to allow flavors to blend. Serve warm or at room temperature.