Place the cream and milk in a small saucepan along with the agar and whisk to combine. Leave for 15 minutes. Lightly brush 2 panna cotta moulds with a little oil.
Add the sugar and vanilla seeds to the mixture and place over medium heat and gently bring to a boil. Simmer for about 3 to 4 minutes, stirring regularly. Remove from the heat and strain through a fine mesh sieve before pouring into the greased moulds. Allow to cool to room temperature before refrigerating for 1 hour.
While the panna cotta is chilling preheat the oven to 180 celsius (350 Fahrenheit) and place the rhubarb, sugar, elderflower cordial and water in a shallow roasting dish. Toss to combine and cover with foil and roast for 15 minutes, or until the rhubarb is tender, but not mushy.
To unmould the panna cotta place the base of the moulds into a dish of hot water for a few seconds before turning out onto a plate or bowl. Serve with the rhubarb and top with the rhubarb roasting liquid.