Preheat your oven to 350°F (175°C). Line an 8- or 9-inch square baking pan with aluminum foil, leaving an overhang. Lightly grease the foil.

Wash and zest the fresh grapefruits, then juice them until you have about 1 cup of juice. Reduce this juice over medium-high heat for 10-12 minutes until it thickens down to about ½ cup.

In a mixing bowl, combine all-purpose flour, granulated sugar, and a pinch of salt. Cut in the butter until the mixture resembles small peas. Press this mixture into the bottom of the prepared pan and bake for 10 minutes.

In another bowl, combine the grapefruit zest with granulated sugar. Mix in the reduction and lemon juice. Gradually whisk in eggs, melted butter, flour, and salt until smooth.

Pour the filling into the pre-baked crust and bake for 18-20 minutes until set with a slight jiggle in the center.

Let the pan cool on a wire rack for about 30 minutes, then refrigerate for at least 4 hours before serving. Dust with confectioner’s sugar before serving.