The slightly bitter greens and bright dressing make this salad the perfect antidote to the winter blues.
Tear the frisee into bite-size pieces, and place in a large bowl with the radicchio; set aside.
Whisk together vinegar and mustard in a small bowl. Season with salt and pepper. Whisking constantly, pour in oil in a slow, steady stream; whisk until emulsified. Just before serving, drizzle vinaigrette over salad; toss gently.