Slice one of the grapefruits in half and squeeze the juice from it into a small bowl.

Slice the onion in half, and then using a sharp knife or mandoline, slice the halves into thin half moons.

Reserve ¼ cup grapefruit juice for the dressing, and toss the remainder with the onion slices. Set aside to marinate while you're preparing the rest of the salad.

Toast the fennel seeds in a dry pan over medium heat until they are just beginning to smell fragrant.

Transfer them into a small bowl, and using the end of a wooden spoon (or a mortar and pestle if you've got that situation going on) bash the fennel seeds to break them up a bit.

Add the reserved ¼ cup grapefruit juice, honey, Dijon, some salt and pepper, and the olive oil, and whisk everything together to form the dressing.

Using a pairing knife, slice the ends from the remaining two grapefruits, then cut all of the remaining peel and pith and outer membranes away.

Slice the grapefruit into thin rounds or half moons.

Trim the fronds from the fennel, slice the bulb in half, and remove the core. Use a very sharp knife or a mandoline to slice paper thin slices of fennel.

When you're ready to serve, drain the onions from the grapefruit juice and place them in a large salad bowl. Add the spinach, grapefruit (you may need to drain this a bit as well), and fennel, then pour the dressing over top and give everything a good toss. Serve immediately.