Preheat oven to 425 degrees. Cut 1 head cauliflower into florets; toss on a large rimmed baking sheet with 3 tablespoons olive oil; season with kosher salt and freshly ground black pepper. Roast, tossing occasionally, until almost tender, 35-40 minutes. Remove from oven and season with salt.
Meanwhile in a high powered blender, combine the basil, tahini, 2 tablespoons olive oil, red wine vinegar, black pepper, and ¼ cup water. Blend on high until you have a smooth, creamy sauce. Plate cauliflower and drizzle with sauce.