Heat oven to 450° F. Cook the barley according to the package directions.
Meanwhile, toss the cauliflower, celery seed, 3 tablespoons of the oil, and ½ teaspoon each salt and pepper on a rimmed baking sheet. Roast, tossing once, until golden brown, 16 to 18 minutes.
Toss together the barley, cauliflower, radicchio, salami, Manchego, scallions, vinegar, the remaining 3 tablespoons of oil, and ¼ teaspoon each salt and pepper in a large bowl.
Serve warm or at room temperature.