Soak gelatin sheets in a bowl of cold water for 5 minutes to let them bloom.
Combine cream and sugar in a small saucepan over medium heat and bring to a simmer. Remove saucepan from heat and set aside.
Lift the gelatin sheets from the cold water and wring gently to remove excess water.
Add the gelatin sheets to the warm cream mixture and stir until they melt completely.
Whisk in the orange blossom water and yogurt until well combined.
Divide the cream mixture among six ramekins. Cover and refrigerate for at least 4 hours or overnight for the panna cotta to set.
Preheat the oven to 350 degrees F.
To prepare the rhubarb, trim the rhubarb at each end and cut them into 1/2 inch batons.
In a baking dish, combine the rhubarb, sugar, and orange juice.
Bake in the oven for 20 minutes until the rhubarb is tender but intact. Let it cool to room temperature.
When ready to serve, unmold the panna cotta by running a sharp knife around the edges of each ramekin to break the suction, place a serving plate over the top of the ramekin, and invert onto the plate. Garnish with the baked rhubarb and chopped pistachios.