Preheat the oven to 350°F (175°C). In a mixing bowl, combine flour, butter, and powdered sugar until crumbly. Add egg yolk and mix until dough forms. Press into tart pan and bake for 15 minutes.

In a saucepan, whisk together lemon juice, sugar, egg yolks, and lemon zest. Cook over medium heat until thickened. Remove from heat and stir in melted butter.

In a clean bowl, beat egg whites until soft peaks form. Gradually add sugar and continue beating until stiff peaks form. Fold in lilac syrup.

Pour the lemon filling into the baked crust. Top with meringue, spreading it to the edges. Bake for an additional 10 minutes or until meringue is lightly browned.