- 1 tablespoon cider vinegar
- 1/2 teaspoon whole-grain or coarse-grain mustard
- 1/4 teaspoon salt
- 1/4 teaspoon black pepper
- 1/4 cup extra-virgin olive oil
- 3 navel oranges
- 2 grapefruit (preferably pink)
- 2 medium fennel bulbs (sometimes called anise), stalks cut off and discarded
- 3 celery ribs, cut diagonally into 1/8-inch-thick slices
- Whisk together vinegar, mustard, salt, and pepper in a large bowl. Add oil in a slow stream, whisking, until combined.
- Cut peel, including all white pith, from oranges and grapefruit with a paring knife. Working over a bowl, cut segments free from membranes. Squeeze juice from membranes into a small bowl, then whisk 3 tablespoons juice into vinaigrette. Drain citrus segments, then add to vinaigrette.
- Quarter fennel bulbs lengthwise, then cut lengthwise into paper-thin slices (about 1/16 inch thick) with slicer. Add to fruit along with celery and toss gently to combine.