In a bowl, combine the flour, 1 teaspoon sugar, lemon zest, and salt. With a fork, blend in the butter until the mixture resembles coarse meal. Add 2 teaspoons of ice water and toss until incorporated. Add remaining ice water, tossing with a fork until the mixture begins to come together. Form the dough into a ball, wrap in plastic wrap, and chill for 30 minutes.
Preheat the oven to 350 degrees. On a lightly floured surface with a floured rolling pin, roll out the dough into a 7 X 4-inch rectangle. Transfer the dough to a baking sheet and poke it all over with a fork. Bake until golden, about 10 minutes. Transfer to a cooling rack set over a baking sheet lined with parchment paper.
Meanwhile, in a small saucepan, stir together 2 tablespoons water and remaining 1/2 cup sugar. Bring the mixture to a boil over moderately high heat, brushing down the sides of the pan with a pastry brush dipped in water. Turn the heat to low and simmer for 10 minutes, or until the caramel is light amber.
Arrange the lemon slices down the center of the pastry and pour the hot caramel over the lemon slices. Let the pastry cool. Using a very sharp knife, cut into 4 pieces and serve.