Place six glass compotes (ideally ¾ cup/180 ml size) or ramekins (see note #1) on a small baking sheet or tray and set aside.
Rhubarb
In a small saucepan, combine the rhubarb, sugar and lemon juice over medium-high heat and stir to combine. Continue to stir for another 5 to 8 minutes, until the sugar starts to melt. Reduce the heat to medium and continue to cook until the rhubarb has broken down and become quite mushy, like a chunky jam.
Divide the rhubarb evenly between the six compotes and set aside.
Panna Cotta
In a small saucepan, warm the gelatin and water over medium heat until the gelatin has dissolved. Remove from the heat.
In another saucepan over medium heat, whisk together the cream, sugar, vanilla paste and salt. Adjust the heat to bring it to a boil. Remove the pan from the heat and whisk in the gelatin mixture until combined.
Divide the panna cotta mixture among the compotes, carefully pouring it atop the rhubarb in each one. Place the panna cotta tray in the refrigerator until they just start to set, about 1 hour.
Graham Topping
In a small bowl, stir to combine the graham crumbs and sugar.
Remove the tray of compotes from the refrigerator and sprinkle the top of each one with approximately ¾ tablespoon of graham crumbs. Return the tray to the refrigerator until the panna cotta has fully set, at least 3 hours or overnight.
Notes
If you have chosen to use ramekins, you will need to release each panna cotta onto a plate to fully appreciate the layers when serving. To do this, simply set the ramekins in a shallow bowl of hot water for a few seconds, making sure the water only rises halfway up the side of the ramekin.
When ready, run a small knife around the edge of the panna cotta and then invert it over a small dessert plate. Gently lift the ramekin to release it.