• Preheat oven to 350 °F. Lightly butter a 13 x 9 baking pan, then line with parchment paper in one direction going up and over the two sides of the pan.

For the Crust

• Combine the flour, sugar and salt in a mixer bowl. On low speed, add the butter. Increase the speed to medium and mix just until the butter is incorporated. It will look powdery, but it will come together when you press it into the pan.
  10 ounces all purpose flour, 8 ounces granulated sugar, ½ teaspoon salt, 8 ounces unsalted butter
• Dump the mix into the pan and press out into an even layer, making sure to get the corners. Bake until golden brown. About 15-20 minutes. If the crust puffs up in any spots, just gently press it down (wearing an oven mitt) after it's out of the oven
• Reduce oven temperature 325 °F.

For the Filling:

• Using the same bowl you used for the crust, whisk together the sugar, salt and flour. Add the eggs and whisk until combined. Add the finely grated zest of 1 blood orange, 1 cup of blood orange juice and the juice from the lemon. Whisk to combine.
  8 ounces granulated sugar, ¼ teaspoon salt, 3 tablespoons all purpose flour, 4 eggs, 6 blood oranges, 1 lemon
• Pour the filling onto the warm crust and bake until the custard is set, about 20 minutes. Give the pan a little shake and if the custard doesn't jiggle, it's ready.
• Cool to room temp in the pan. Run a butter knife between the parchment and the pan to make sure the filling didn't leak out and cause the paper to stick. Cool completely.
• Use the parchment paper to lift the bars onto a cutting board. Peel back the edges of the paper and slide the paper out from under the crust. Cut into 24 squares. Wipe the blade of the knife with a damp cloth between cuts to get clean edges. Sift an even layer of confectioner's sugar over the surface. Transfer to a serving plate.