Peppery watercress and pleasantly bitter radicchio stand up well to the lively country mustard vinaigrette in this simple salad. Radishes add a welcome crunch.
In a salad bowl, whisk together lemon juice, Dijon, and sugar; season with salt and pepper. Slowly add oil, whisking constantly, until emulsified. Add watercress, radicchio, and radishes. Toss to evenly coat in dressing; serve.
For a more festive-looking salad, use a variety of radicchios, such as Treviso, radicchio variegato, and radicchio rosa.