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1
Grate the fruit peel on the second-finest blade of a box grater.
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2
Then, juice the fruit, yielding 2 1/2 cups of juice.
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3
Place 3/4 cup of the juice, all but 1 teaspoon of the rind, and the sugar in a small saucepan.
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4
Sprinkle the gelatin over the juice, and allow to sit for 1 to 2 minutes, or until the gelatin is no longer white.
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5
Over medium heat, bring the liquid just to a simmer.
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6
Lower heat slightly, and cook, stirring, for 5 minutes.
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7
Strain the juice and gelatin mixture into a medium stainless-steel bowl over a larger bowl filled with ice and water.
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8
Stir in the remaining juice; whisk to combine.
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9
Stirring from time to time with a rubber spatula, allow the gelatin to become firm, but not completely set.
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10
This process should take about 20 to 25 minutes.
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11
Meanwhile, whip the heavy cream to medium peaks.
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12
When the gelatin is firm, beat it slightly with a wire whisk to lighten.
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13
Beat in 1 1/2 cups of the whipped cream with a rubber spatula until fully combined, but not over-mixed.
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14
Smooth into the prepared tart shell, mounding slightly toward the center.
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15
Top with additional whipped cream, saving 1/2 cup for garnish if desired.
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16
Sprinkle the reserved teaspoon of zest over the top.
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17
Refrigerate for 1 to 2 hours, or until set.
