Toast the pistachios in a dry skillet over medium heat, stirring, until fragrant and lightly colored, 4–6 minutes; cool, then coarsely chop and set aside.
Trim the fennel bulb, remove the tough outer layer, core, and slice very thinly (a mandoline helps); place in a large mixing bowl.
Thinly slice the red onion and soak in cold water for 10 minutes to mellow; drain well and add to the bowl.
Working over a separate bowl, cut the peel and pith from the navel oranges and segment them by cutting between the membranes.
Segment the blood oranges over the same bowl to catch any juices.
Segment the grapefruit as well, keeping the pieces intact; discard seeds.
Roughly tear the mint leaves and add to the mixing bowl with the vegetables.
In a small bowl, whisk the lemon juice, maple syrup, kosher salt, and black pepper until dissolved; slowly stream in the extra-virgin olive oil while whisking to emulsify.
Add the citrus segments to the mixing bowl, pour over the dressing, and gently toss to coat without breaking the fruit; fold in half the nuts.
Transfer to a platter and finish with the remaining nuts; taste and adjust seasoning, then serve right away.