For the dressing: In a large bowl, combine the yogurt, mayonnaise, fennel fronds, tarragon, honey, mustard, vinegar and cayenne. Season with salt and pepper; mix well. Cover and refrigerate while you prepare the slaw.
For the slaw: In a dry saute pan over medium-high heat, toast the almonds until golden brown and fragrant, about 5 minutes. Shake the pan to ensure even cooking. Set aside.
In a large mixing bowl, combine the broccoli, the green and red cabbage, and the fennel, carrot and onion. Toss well to combine. Add the dressing and toss to coat the vegetables thoroughly. Cover and refrigerate for 25 to 30 minutes to allow the vegetables time to soften.
Sprinkle with the toasted almonds just before serving.