Preheat oven to 320°F (160°C).
Spread rhubarb on a parchment-lined baking sheet. Add sugar, vanilla paste, orange zest, and orange juice, then toss to coat. Halve very thick stalks lengthwise if needed.
Bake the rhubarb for about 20 minutes, checking after 18 minutes, until soft but still holding its shape.
Cool the rhubarb on the baking sheet.
Trace the bottom of the springform pan onto parchment and cut out the circle. Arrange rhubarb stalks side by side, place the parchment circle on top, and cut the rhubarb into a round topping.
Cut the leftover rhubarb into pieces and set aside for the cheesecake filling. Cover and refrigerate the rhubarb circle.
Mix graham cracker crumbs, brown sugar, cinnamon, and salt. Stir in melted butter.
Press the crumbs into the bottom and halfway up the sides of the springform pan.
Bake the crust at 320°F (160°C) for 12 minutes.
Stir together sugar and cornstarch in a large bowl or stand mixer.
Beat in cream cheese until creamy.
Add sour cream, vanilla extract, and salt. Beat well and scrape down the bowl.
Add eggs one at a time, mixing only until each egg is incorporated. Scrape the bowl between eggs and do not overmix.
Pour half the cheesecake batter into the baked crust.
Add the roasted rhubarb pieces evenly over the batter.
Spoon the remaining cheesecake batter on top.
Place the springform pan into a water bath setup with a larger pan and boiling water.
Bake at 320°F (160°C) for 1 hour 30 minutes, until the edges are set and the center is puffed but still jiggly.
Turn off the oven and leave the cheesecake inside for 30 minutes.
Remove the cheesecake, cool at room temperature, then refrigerate for at least 6 hours or overnight.
Run a knife around the cheesecake and remove the ring.
Arrange the rhubarb disk on top, spoon over any leftover syrup, and decorate with edible flowers if desired.
Refrigerate until ready to slice and serve. Use a sharp knife and sawing motion to cut through the rhubarb topping.