How to Make It
Heat 2 tablespoons of the oil in a medium skillet over medium heat. Add the shallots and ½ teaspoon salt and cook, stirring often, until softened, 8 to 10 minutes. Add the vinegar, sugar, the remaining ⅓ cup of oil, ½ teaspoon salt, and ¼ teaspoon pepper and cook, stirring, until heated through, 1 minute.
In a large bowl, combine the frisée and warm vinaigrette and toss to coat.