Preheat your oven to 350°F (175°C) and prepare an 11x7-inch baking dish by greasing it lightly or lining it with parchment paper.

Combine all-purpose flour, powdered sugar, and salt in a food processor. Add room temperature butter cut into pieces and pulse until it resembles coarse crumbs.

Transfer the mixture into the prepared baking dish and press it down evenly. Bake for 18-20 minutes, until golden brown.

Whisk together granulated sugar and a tablespoon of flour in a bowl. Add eggs one at a time, mixing well. Incorporate grapefruit zest and juice.

Pour the grapefruit filling over the hot crust and smooth the top. Bake for an additional 20 minutes until the filling is set.

Allow to cool at room temperature for about an hour, then refrigerate for 1-2 hours before slicing and serving.