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°. Combine first 4 ingredients in a shallow 2-qt. 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 Tbsp. 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.