Nutrition per serving may change if servings are adjusted.
5 pounds cauliflower, cut into 1-inch florets (about 18 cups)
2 large leeks, white and pale green parts only, halved lengthwise, rinsed and cut into ½-inch pieces
1½ cups extra-virgin olive oil, divided
1¾ teaspoons kosher salt, divided
1¾ teaspoons ground pepper, divided
2½ cups fresh parsley
⅔ cup fresh chives
12 cups low-sodium "no-chicken" broth or chicken broth
5 teaspoons white-wine vinegar
Preheat oven to 400°F. Coat 2 large rimmed baking sheets with cooking spray.
Toss cauliflower and leeks with ½ cup oil and 1¼ teaspoons each salt and pepper in a very large bowl (you may need to do this in 2 batches). Divide the vegetables evenly between the prepared baking sheets. Roast, switching the pans from top to bottom and back to front halfway, until soft and browned on the bottom, 25 to 30 minutes.
Meanwhile, place parsley, chives and the remaining ½ teaspoon each salt and pepper in a blender; pulse several times to chop, scraping down the sides once or twice. With the motor running, slowly add the remaining 1 cup oil and process until smooth. Transfer to a bowl and rinse out the blender.
Transfer the roasted vegetables to a large pot and add broth. Bring to a boil over high heat. Reduce heat, cover and simmer for 10 minutes. Puree the soup in batches in a blender. (Use caution when pureeing hot liquids.) Stir in vinegar. Serve with some of the herb sauce swirled on top.
To make ahead: Refrigerate for up to 3 days or freeze for up to 3 months.
329 calories;29 g fat(4 g sat); 4 g fiber; 16 g carbohydrates; 4 g protein; 135 mcg folate; 0 mg cholesterol; 6 g sugars; 0 g added sugars; 1,725 IU vitamin A; 100 mg vitamin C; 76 mg calcium; 2 mg iron; 482 mg sodium; 621 mg potassium
Vitamin C (167% daily value), Folate (34% dv), Vitamin A (34% dv)