Make the crust: Whisk the flour, confectioners' sugar and salt in a medium bowl. Work in the butter with your fingers until a soft dough forms. Firmly press the dough into the bottom and up the sides of a 9-inch tart pan with a removable bottom. Prick the crust in a few spots with a fork. Refrigerate or freeze until firm, 10 to 30 minutes.
Preheat the oven to 350˚ F. Line the dough with parchment or foil and fill with pie weights or dried beans. Bake until the edges are set, about 20 minutes. Remove the parchment and weights and continue to bake until the crust is dry and lightly browned, 10 to 15 more minutes. Transfer to a rack to cool completely.
Make the filling: Combine 3 cups cranberries, 1 1/2 cups granulated sugar and 1/4 cup water in a blender. Remove 5 wide strips of zest from the lemon using a vegetable peeler; set the strips aside. Fully peel the lemon, removing any white pith. Add the peeled lemon and 2 strips of zest to the blender and blend until smooth (reserve the remaining 3 strips of zest for topping). Strain through a fine-mesh sieve into a medium saucepan, pressing on the seeds and skins with a rubber spatula. Bring to a simmer over medium heat, stirring occasionally.
Meanwhile, whisk the whole eggs, egg yolk, cornstarch and vanilla in a medium bowl. Whisk in about 1/2 cup of the hot cranberry mixture, then pour into the saucepan with the remaining cranberry mixture, whisking. Continue to cook, stirring constantly with a rubber spatula, until the filling boils and is thick like pudding, 10 to 15 minutes. Pour into the crust, cover with plastic wrap or an inverted plate and refrigerate until set, at least 4 hours or overnight.
Make the toppings: Bring 1/2 cup each water and granulated sugar to a boil in a small saucepan. Add the remaining 1/2 cup cranberries and 3 strips of lemon zest, then remove from the heat and let cool. Strain through a fine-mesh sieve. Thinly slice the zest. Put the remaining 1/4 cup granulated sugar in a shallow dish, add the cranberries and zest and gently toss to coat. Let set, 30 minutes to 1 hour, shaking occasionally to re-coat.
Beat the heavy cream and confectioners' sugar in a large bowl with a mixer on medium-high speed until medium peaks form. Top the tart with the sugared cranberries and lemon zest. Serve with the whipped cream.