Quarter 2 pounds of the strawberries and set aside. Dice half of the remaining pound, then slice the remainder.
Stir 1 tablespoon granulated sugar together in a bowl with the elderflower liqueur until the sugar dissolves. Toss with the diced berries and set aside to macerate at room temperature.
Place the quartered strawberries in a medium pot and gently stir together with the jam, cornstarch, lemon juice, a pinch of salt and the remaining 1/3 cup granulated sugar. Bring to a simmer over medium heat and cook, stirring often, until the strawberries begin to break down and the mixture is thick and jammy, 15 to 20 minutes. Remove from the heat and let cool about 10 minutes, then stir in the reserved macerated berries.
Pour mixture into the baked crust and allow to chill in the refrigerator until set, at least 3 hours.
Decorate the top of the pie with the sliced berries in concentric circles. Refrigerate until ready to serve.
Whip the cream along with the confectioners' sugar, vanilla and cream of tartar until it holds medium stiff peaks. Serve alongside the pie.