Peel the rhubarb with a knife and cut them into 15 cm long pieces. Place the pieces in a vacuum bag. Juice the trimmings in a cold press juicer. Bring the juice to a boil with birch sap, sugar and fennel seed and cook until the seeds are soft. Strain the liquid and pour it into the bag with the rhubarb, keep the fennel seeds in a bowl. Seal the bag and cook for 5-10 minutes in a water bath or steam oven at 80°C. Let the rhubarb rest in the bag for 10 minutes, then pour the liquid into a pan and place the rhubarb on a tray. If you do not have access to a vacuum machine you can simmer the rhubarb in the juice on the stovetop instead.
Frosted fennel seed
Toss the fennel seed in the sugar making sure all seeds are covered. Spread the seeds on a tray and let them dry in room temperature.
Boil the liquid with agar agar until the liquid thickens. Peel the rhubarb and dice it finely. Mix the rhubarb with the liquid and leave to cool in room temperature.
Whip all ingredients to a firm meringue. Spread it thinly onto silicone sheets and dry it in the oven at 90°C. Take out the meringue while still chewy and tear it into pieces as instructed by your chef. Burn the pieces with a blowtorch and leave to cool on a tray.
Buttermilk ice cream
Boil the buttermilk with sugar, glucose and agar agar. Until the sugar has melted. Add the egg yolks and whisk over medium heat until the mixture reaches 84°C. Remove the pan from the heat and add the cream.
Combine and knead all ingredients to form a dough. Press the dough out flat onto a silicone sheet and bake in the oven at 170 °C for 15 minutes or until the cake is golden brown. Crush the cake to a fine crumble.
Slice the rhubarb thinly using a mandolin and place the slices in cold water. Pluck the herbs, cover them with wet paper and keep in the fridge until serving