This salad combines bitter and spicy lettuces with the sweetness of winter oranges and the tart flavor of red-wine vinegar.
In a small bowl or jar, whisk or shake orange juice, vinegar, mustard, and oil until thickened and combined; season with salt and pepper. (Dressing can be made ahead and refrigerated, covered, until ready to serve.)
In a large bowl, combine arugula, endive, and oranges; toss with dressing. Serve immediately.