1 large head cauliflower (3 to 3 1/2 punds), cut into 1-inch-wide florets (9 cups)
1/2 cup plus 1 tablespoon extra-virgin olive oil
3/4 teaspoon salt
1/4 teaspoon black pepper
1/4 cup white-wine vinegar
1 1/2 tablespoons finely chopped shallot
2 heads romaine (2 pounds total), cut crosswise into 1/4-inch-wide strips
1 large head radicchio (3/4 pound), cut crosswise into 1/4-inch-wide strips
1 cup loosely packed fresh flat-leaf parsley leaves (from 1 bunch)
1/2 cup hazelnuts (2 1/4 ounces), toasted , any loose skins rubbed off in a kitchen towel, and nuts coarsely chopped
Put oven rack in middle position and preheat oven to 450°F.
Toss cauliflower with 1/4 cup oil, 1/2 teaspoon salt, and 1/8 teaspoon pepper in a large bowl. Spread in 1 layer in a shallow baking pan (1 inch deep) and roast, turning over with tongs halfway through roasting, until tender and golden brown, 25 to 30 minutes total. Cool in pan on a rack, then transfer to large bowl.
Whisk together vinegar, shallot, remaining 1/4 teaspoon salt, and remaining 1/8 teaspoon pepper in a small bowl, then add remaining 5 tablespoons oil in a slow stream, whisking until emulsified. Add half of dressing to cauliflower and toss to coat. Add romaine, radicchio, parsley, half of nuts, and remaining dressing to cauliflower and toss to coat. Season with salt and pepper and sprinkle with remaining nuts.