Special equipment: a 9-inch fluted tart pan
Add the eggs and cream to a medium bowl and whisk together until combined.
In a saucepan over medium heat, add the sugar, clementine juice, grapefruit juice and cornstarch. Cook, whisking, until the sugar dissolves, 3 to 5 minutes. Slowly add the egg-cream mixture and stir to combine. Cook, stirring continuously, until the mixture thickens enough to coat the back of a spoon, 8 to 10 minutes.
Pour the citrus custard into the prebaked Shortbread Almond Crust and refrigerate until set, about 4 hours or overnight.
Remove the tart from the refrigerator, top evenly with Citrus Whipped Cream, and sprinkle with the citrus zest.
Preheat the oven to 350 degrees F.
Place the cookies and almonds into a food processor and pulse to chop fine. Add the sugar, cinnamon and salt and pulse to combine. Drizzle in the melted butter and process until the mixture holds together.
Press the crust into a 9-inch fluted tart pan, evenly covering the bottom and up the sides of the pan. Refrigerate for 20 minutes, then bake until golden and crisp, 18 to 20 minutes.
Cool completely on a wire rack before filling.
In a large bowl, whip the heavy cream, confectioners' sugar and 1 tablespoon of the citrus zest until stiff peaks form.