Prepare the fennel: Trim the fennel bulb by cutting off the stems and removing any outer layers. Slice it thinly, either with a knife or a mandoline. Place the sliced fennel in a large bowl.
- Prepare the grapefruit: Peel the grapefruits with a knife, removing all the pith. Cut into segments by following the natural lines of the fruit. Be sure to collect any juices that escape while segmenting. Add the grapefruit segments to the bowl with fennel.
- Make the dressing: In a small bowl, whisk together the olive oil, white wine vinegar, honey, Dijon mustard, salt, and pepper until smooth. Taste and adjust the seasoning as needed.
- Assemble the salad: Drizzle the dressing over the fennel and grapefruit mixture, and toss gently to combine.
- Top with goat cheese: Sprinkle the crumbled goat cheese over the salad and toss again gently, or leave it on top for a more decorative presentation.
- Garnish: If desired, sprinkle fresh parsley on top for a burst of color and flavor.
- Serve: Serve the salad immediately, or refrigerate for up to 30 minutes before serving to allow the flavors to meld together.