Preheat oven to 450°F. Combine first 4 ingredients in a shallow 2-quart baking dish; toss to coat. Season with salt and pepper.
Roast vegetables, stirring occasionally, until onions are caramelized and softened, about 40 minutes. Stir in 1 tablespoon Sherry vinegar. Season to taste with salt, pepper, and more vinegar, if desired.
Arrange frisée on plates, dividing evenly. Spoon vegetables over frisée. Drizzle juices in baking dish over. Top with cheese, if desired.
Per serving: 190 calories, 15 g fat, 2 g fiber Nutritional analysis provided by Bon Appétit