Adjust oven rack to lower-middle position and preheat to 350 degrees F. Combine pistachios and lemon syrup in a small bowl, stirring to coat. Spoon onto a rimmed aluminum baking sheet lined with parchment . Toast pistachios until deep amber in color, 12 to 15 minutes; the acidic syrup browns dramatically, which makes the pistachios difficult to judge by eye, but don't mistake this for burning.
