Preheat oven to 350 degrees F. Toss cauliflower in 1 tablespoon oil and spread in a single layer in a large baking dish. Season with salt and pepper. Roast, turning with a spatula halfway through, until the florets are golden brown and slightly crisp on the outside, about 30 minutes.
In a large saucepan over medium heat, toast the pine nuts, stirring frequently, until fragrant and golden - about 5 minutes. Set toasted pine nuts aside and return the saucepan to the heat. Add barley and enough water to cover by two inches.
Season with salt. Bring to a boil and cook until tender, 25-30 minutes. Drain; run under cold water. Set aside.
Meanwhile, whisk lemon juice, Dijon mustard, and 5 tablespoons oil in a medium bowl until emulsified. Season dressing with salt and pepper; set aside.
In a large bowl combine cauliflower, barley, beans, pine nuts, 1/4 cup parsley, 1 tablespoon tarragon, and half of reserved dressing. Toss to coat; season with salt and pepper.
Divide salad among bowls; drizzle remaining dressing over. Garnish with lemon zest, 1/4 cup parsley, and 1 tablespoon tarragon.