Preheat your oven to 350°F (175°C). In a mixing bowl, combine 1 cup of all-purpose flour, 1/4 cup of granulated sugar, and 1/2 cup of softened unsalted butter. Mix until the mixture resembles coarse crumbs. Press this mixture evenly into the bottom of an 8-inch square baking dish.

Place the baking dish in the preheated oven and bake for 10–15 minutes, until the edges are lightly golden and firm. Let it cool slightly while preparing the filling.

In a large mixing bowl, whisk together 2 eggs and 1 cup of granulated sugar until smooth. Gradually add in 3/4 cup of fresh grapefruit juice, 2 tablespoons of grapefruit zest, and 2 tablespoons of all-purpose flour. Mix until well combined.

Carefully pour the grapefruit filling over the pre-baked crust, spreading it evenly.

Return the baking dish to the oven and bake for an additional 25–30 minutes, until the filling is set and slightly golden around the edges.

Remove from the oven and allow to cool at room temperature for at least an hour. Cover with plastic wrap and refrigerate for at least 2 hours before serving.