Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan (or line with parchment paper).
Step 2: In a large mixing bowl, whisk together the flour, granulated sugar, baking soda, cinnamon, nutmeg, and salt. Set aside.
Step 3: In a separate medium bowl, whisk together the vegetable oil, eggs, vanilla extract, and 1/4 cup fresh orange juice until well combined.
Step 4: Pour the wet ingredients into the dry ingredients. Mix with a spatula or wooden spoon until just combined. Be careful not to overmix.
Step 5: Fold in the finely grated carrots, 1/2 cup chopped pistachios, and orange zest until evenly distributed.
Step 6: Pour the batter into the prepared cake pan and spread evenly. Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean.
Step 7: Remove the cake from the oven and let it cool in the pan on a wire rack for 10-15 minutes. Then, carefully invert the cake onto the wire rack to cool completely.
Step 8: While the cake cools, prepare the rose glaze. In a small bowl, whisk together the powdered sugar, 2-3 tablespoons fresh orange juice, and rosewater until smooth and pourable. Adjust consistency with more juice or powdered sugar if needed.
Step 9: Once the cake is completely cool, place it on the wire rack over a baking sheet. Drizzle the rose glaze evenly over the top of the cake, allowing it to drip down the sides. Immediately sprinkle with additional chopped pistachios and dried rose petals, if desired.
Step 10: Let the glaze set for at least 15-20 minutes before slicing and serving. Store any leftover cake in an airtight container at room temperature for up to 3 days, or in the refrigerator for up to 5 days.