Trim the fennel bulbs, removing any tough outer layers and the stalks (reserve the fronds if using). Slice the fennel thinly, either using a mandoline or a sharp knife. If you'd like a milder flavor, you can soak the fennel in ice water for 10 minutes before draining.

Peel the oranges and grapefruit with a sharp knife, cutting away the white pith. Segment the fruit by slicing between the membranes, catching any juices. Set the segments aside and reserve the juice for the dressing.

Thinly slice the red onion. You can soak it in cold water for a few minutes to mellow the sharpness if desired.

In a small bowl, whisk together the olive oil, vinegar, honey (if using), Dijon mustard, and the reserved citrus juice. Season with salt and pepper to taste.

In a large bowl, combine the sliced fennel, citrus segments, red onion (if using), and chopped herbs (parsley, mint, and fennel fronds).

Drizzle the dressing over the salad and toss gently to combine. Taste and adjust seasoning if necessary.

Serve the salad chilled or at room temperature. It pairs wonderfully with grilled fish, roasted chicken, or as a light side to any Mediterranean meal.