How to Make It
Preheat the oven to 300°. In a large roasting pan, toss all but 1 cup of the rhubarb with the orange juice, elderflower concentrate, sugar, vanilla and 1 cup of the water. Place a crumpled sheet of parchment or wax paper directly on the rhubarb, then cover the roasting pan tightly with foil. Bake the rhubarb for 1 1/2 hours, or until tender.
Strain the cooked rhubarb through a fine-meshed sieve set over a bowl, pressing lightly on the solids to extract as much juice as possible; there should be 4 cups of juice. Reserve the mashed rhubarb for another use.
Meanwhile, blanch the remaining 1 cup of rhubarb in boiling water until just tender, about 5 minutes. Transfer the rhubarb to a plate to cool.
Pour the remaining 1/4 cup of cold water into a small bowl. Sprinkle the gelatin over the water and let stand for 10 minutes, or until the gelatin is completely softened.
Heat 1/2 cup of the rhubarb juice in a small saucepan. Add the gelatin mixture and cook over low heat, stirring until the gelatin is completely dissolved. Stir the mixture into the remaining 3 1/2 cups of rhubarb juice and let cool slightly. Pour the jelly into 8 glasses and refrigerate until slightly set but still very jiggly. Stir the blanched rhubarb into each glass, being careful not to create too many bubbles, and refrigerate until firm, at least 2 hours.