Peel the rhubarb stalks and cut them to uniform lengths. Place them in an earthenware dish with the caster sugar to macerate for 2 hours, turning occasionally, then drain.
Preheat the oven to thermostat 7 (210°C). Bundle the stems with raffia and arrange in a buttered baking dish. Sprinkle with one-third of the vergeoise sugar and bake.
Meanwhile, prepare a compote using the maceration juice and remaining rhubarb pieces.
After 20 minutes, turn the bundles over. Sprinkle with another third of the vergeoise, dot with half the butter, and continue baking for 30 minutes, basting with juice.
Turn the bundles once more, sprinkle with the remaining vergeoise, add the rest of the butter, and switch to the grill.
Caramelize using a branding iron or kitchen torch, then serve warm with the compote.
