In a small bowl, measure 2/3 cup grapefruit juice and 2 tablespoons lemon juice with half a sprig of rosemary soaking in it. Set aside.

Preheat the oven to 350 degrees F. Grease an 8 x 10 pan with butter.

For the crust: Stir together the flour, sugar, and remaining rosemary (chopped finely). Add the butter to the bowl and use a pastry cutter to cut it all together until the mixture resembles fine crumbs. Press into the prepared pan and bake until golden around the edges, about 15-20 minutes.

For the filling: Stir together the sugar and flour. Crack in the eggs and whisk to combine. Add the infused grapefruit juice and mix until combined. Pour over the crust and bake about 20-25 minutes, or until it is set and the middle is no longer jiggling.

Allow to cool in the fridge for 2 hours at the least, then sift powdered sugar over the top before cutting into squares. Enjoy!