Toast the pistachios in a dry skillet over medium heat for 3–5 minutes, stirring frequently, until fragrant. Transfer them to a plate and let them cool before roughly chopping.

Cut the tops and bottoms from the blood oranges. Stand each orange upright and slice away the peel and white pith with a sharp knife, then cut between the membranes to remove the segments over a bowl so the juice is captured.

Measure 1 tablespoon of the collected blood-orange juice for the dressing. Keep the orange segments aside while preparing the fennel and vinaigrette.

Trim the fennel bulb, reserving some green fronds. Halve the bulb and slice it as thinly as possible crosswise or lengthwise; a mandoline creates the fine, crisp slices traditionally used in this salad.

Whisk the olive oil, lemon juice, reserved blood-orange juice, lemon zest, honey or maple syrup, Dijon mustard, salt, and pepper until emulsified.

Combine the fennel, red onion, mint, and chopped fennel fronds in a serving bowl. Add about two-thirds of the dressing and toss gently.

Arrange the blood-orange segments over the fennel, spoon over the remaining dressing, and finish with the toasted pistachios, salt, and black pepper. Serve immediately, or refrigerate the dressed fennel for up to about 30 minutes before adding the orange segments and pistachios.