- 2 medium red onions, quartered through stem end
- 6 roasted red peppers (from a 24-ounce jar, preferably packed in brine), drained, halved lengthwise
- 6 garlic cloves
- 1/4 cup extra-virgin olive oil
- Kosher salt, freshly ground pepper
- 1 tablespoon (or more) Sherry vinegar or red wine vinegar
- 8 cups (loosely packed) frisée
- 1/3 cup thinly sliced Spanish blue cheese (such as Cabrales or Valdeón; optional)
- 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.