In a large bowl, whisk together the flour and salt. Add the cold butter cubes and use a pastry blender or your fingertips to cut the butter into the flour until the mixture resembles coarse crumbs. Gradually add the ice water, one tablespoon at a time, mixing until the dough just comes together. Be careful not to overmix! Form the dough into a disk, wrap it in plastic wrap, and refrigerate for at least 30 minutes. This chilling period is crucial for a flaky crust.

On a lightly floured surface, roll out the chilled dough into a 12-inch circle. Carefully transfer the dough to a 9-inch tart pan with a removable bottom. Press the dough into the bottom and up the sides of the pan. Trim any excess dough from the edges and crimp the edges for a decorative finish.

Preheat your oven to 375°F (190°C). Line the tart shell with parchment paper and fill it with pie weights or dried beans. This prevents the crust from puffing up during baking. Bake for 15 minutes, then remove the parchment paper and weights. Bake for another 5-7 minutes, or until the crust is lightly golden brown. Let it cool completely.

While the crust is cooling, prepare the grapefruit filling. Zest one grapefruit (you should have about 1 tablespoon of zest) and set aside. Peel all the grapefruits and segment them, removing any membranes. In a medium bowl, whisk together the eggs, sugar, heavy cream, rosemary, grapefruit zest, and vanilla extract until well combined.

Arrange the grapefruit segments evenly over the bottom of the cooled tart shell. Pour the egg mixture over the grapefruit segments, making sure to distribute it evenly. Bake in the preheated oven for 30-35 minutes, or until the filling is set and slightly golden brown. The center should still have a slight wobble.

Let the Grapefruit and Rosemary Tart cool completely on a wire rack before serving. This allows the filling to set properly. Once cooled, you can remove the tart from the pan by gently lifting the bottom.