Top this red-grapefruit-and-fennel salad with a citrus-based dressing, such as Green Herb Sauce or Creamy Lemon Buttermilk Vinaigrette.
Using a paring knife, remove peel, pith, and outer membranes from 2 grapefruit. Slice into 1/4-inch rounds; set aside. Squeeze juice from remaining grapefruit, and set aside.
Place fennel seeds in a skillet over medium-high heat. Cook until golden brown and fragrant, about 3 minutes. Transfer to a spice grinder, and grind until very fine.
Place ground seeds in a bowl; add 1/4 cup grapefruit juice, honey, and mustard. Season with salt and pepper. Whisk to combine, slowly adding olive oil. Set dressing aside.
Using a mandoline or sharp knife, slice onion into thin rounds; place in a bowl. Pour remaining grapefruit juice over onions; toss.
Cut fennel in half lengthwise; remove core. Using mandoline, slice fennel. Drain onions; place in a large bowl. Add fennel, spinach, and grapefruit rounds. Toss with dressing.