Heat the butter in a heavy bottom pan, then add the rice and allow it to cook for one minute. Add 25 g of sugar and continue cooking on low heat, until the rice is coated in sugar and starts to caramelise gently. Add 200 ml of milk and 75 g sugar and stir. Continue cooking on very low heat, gradually adding the rest of the milk and stirring occasionally.
Make a sugar syrup by boiling 250 ml water with 250 g sugar. When the syrup begins to boil, add the rhubarb cut in sticks of about 7cm long. Once the rhubarb is added, take the pan off the heat and allow the rhubarb to gently cook while the liquid is cooling.
To serve, place the rice pudding into bowls and add rhubarb on top. Garnish with crushed pistachio and lavender (optional).