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1
Preheat the oven to 300.
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2
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.
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3
Place a crumpled sheet of parchment or wax paper directly on the rhubarb, then cover the roasting pan tightly with foil.
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4
Bake the rhubarb for 1 1/2 hours, or until tender.
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5
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.
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6
Reserve the mashed rhubarb for another use.
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7
Meanwhile, blanch the remaining 1 cup of rhubarb in boiling water until just tender, about 5 minutes.
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8
Transfer the rhubarb to a plate to cool.
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9
Pour the remaining 1/4 cup of cold water into a small bowl.
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10
Sprinkle the gelatin over the water and let stand for 10 minutes, or until the gelatin is completely softened.
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11
Heat 1/2 cup of the rhubarb juice in a small saucepan.
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12
Add the gelatin mixture and cook over low heat, stirring until the gelatin is completely dissolved.
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13
Stir the mixture into the remaining 3 1/2 cups of rhubarb juice and let cool slightly.
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14
Pour the jelly into 8 glasses and refrigerate until slightly set but still very jiggly.
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15
Stir the blanched rhubarb into each glass, being careful not to create too many bubbles, and refrigerate until firm, at least 2 hours.
