Place the fennel seeds in a small skillet and toast them over medium heat until they turn a shade darker and are fragrant. Let them cool, then grind or pound them coarsely. Place the crushed fennel seeds in a bowl with the olive oil and let steep for 30 minutes.
Heat the oven to 400° F (200° C). Cut the carrots in half lengthwise and place them on a rimmed baking sheet. Drizzle with the fennel-infused oil and season them lightly with salt and pepper. Roast until the carrots are tender and lightly browned, 25 to 30 minutes.
In a small bowl, whisk together the honey and vinegar. Drizzle this over the carrots, turning them to coat them. Sprinkle the pistachios on top and roast until the carrots are nicely caramelized, about 5 minutes longer. (You can pop them under broiler for a minute or so to caramelize them a bit more if you like, but take care not to let them burn.)
Arrange the carrots on a plate, sprinkle with a little more salt, and serve.
