Preheat the oven to 375 degrees and grease a 7” x 11” pan. Set the pan aside.
First combine the sauce ingredients in a medium saucepan and using a whisk, stir occasionally. Bring the ingredients to a boil at a medium heat. Boil for 5 minutes and then remove from the heat and set aside.
Next, in a large bowl add the flour and salt into a bowl. Then cut the cubed, cold, butter into the flour as when you make pie crust, using a pastry blender or fork. Be sure to work the butter into the dough until it resembles small crumbs.
Then measure out the sourdough starter and cold milk in a measuring cup and mix it together with a fork until combined. You could also do this in a small bowl if you prefer. Start with ¼ cup milk and then add as needed once you add it to the flour/butter mixture.
Once the starter and milk are combined add this to the pie dough dry ingredients and butter. Mix until just moistened. You don’t want to overwork the dough. Once it’s mixed remove the dough to a clean, lightly floured surface.
Make a disc out of the dough flattening it and adding flour to the surface as needed to prevent it from sticking. Roll out the dough into a big rectangle. You want to roll the dough thin. (This will result in a flakier crust.) Once it is thinly rolled out using a knife or bench scraper square up the edges and then divide the crust evenly into 6 pieces. Cutting the long side into 3 equal pieces.