Position rack in lower third of oven; preheat to 450°F. Coat a large rimmed baking sheet with cooking spray.
Cut cauliflower into quarters. Remove any extra woody core from the ends, but keep the quarters intact. Brush with 2 teaspoons oil and sprinkle with 1/8 teaspoon salt. Place cut-side down on the prepared baking sheet.
Roast the cauliflower for 15 minutes. Turn so the opposite cut sides are down. Continue roasting until tender, 15 to 20 minutes more.
Meanwhile, combine the remaining 1 tablespoon oil, the remaining 1/4 teaspoon salt, blue cheese, vinegar, water, scallion greens and pepper in a small bowl. Serve the roasted cauliflower drizzled with the vinaigrette.
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