Place the pistachios in a food processor and blend until roughly chopped. You can also use a knife to do so. Mix the pistachios with 2 tbsp sugar and 2 tsp rosewater, set it aside.
Place 2 sheets of phyllo dough in the springform pan with the edges hanging. Brush the phyllo dough with melted butter (including the edges). Rotate the pan 90 degrees and place another 2 sheets of phyllo dough with the edges hanging. Brush with melted butter again. Rotate 90 degrees and repeat with the remaining phyllo sheets, brushing every 2 sheets with melted butter. You should have every part of the pan covered with phyllo sheets and the edges hanging.
Spread the pistachio mix on the phyllo sheets in the bottom of the pan. Set it aside.
Preheat the oven to 350 degrees F.
Place the cream cheese, ricotta, rosewater and granulated sugar in a bowl. Mix using an electric mixer on medium low speed until all is combined.
Add in the eggs and heavy cream to the cheese mixture and mix again. Finish with the cornstarch and mix slowly one last time.
Pour in the batter into the springform pan. Bake in the oven for 50 minutes. until the phyllo dough is golden and crispy. The cheesecake should be jiggly in the middle.
Take the cheesecake out of the oven and let it sit for 10 minutes. Slowly break the crispy overhanging phyllo pieces and crumble them on the edges of the cheesecake. Spoon the cool simple syrup all around the edges on the crispy phyllo. You might not need all the syrup.
Once the cheesecake has cooled completely, transfer to the fridge and refrigerate for 8 hours or overnight.
Open the springform pan and transfer to a serving plate or cake stand. Top with ground pistachio and edible roses if using. Slice and serve.