Step 1: Preheat your oven to 400°F (200°C). Place the nectarine halves cut-side up in a medium baking dish.
Step 2: In a small bowl, whisk together the honey and the seeds scraped from the vanilla bean. Drizzle this mixture evenly over the nectarines.
Step 3: Place the nectarines in the oven and roast for 10 minutes until they begin to soften and release their juices.
Step 4: While the fruit roasts, combine the pistachios, oats, flour, butter, salt, and cinnamon in a small bowl. Use your fingers to rub the butter into the dry ingredients until it forms a coarse, pebbly crumble.
Step 5: Remove the baking dish from the oven. Carefully spoon the pistachio crumble into the center of each nectarine half and sprinkle the remaining crumble over the top.
Step 6: Return the dish to the oven and bake for another 10-12 minutes, or until the topping is golden brown and the fruit is tender and bubbling. Serve warm.