• Preheat the oven to 375ºF. Roll out the crust and fit into a 9-inch pie pan. Partially bake the unbaked pie crust without the filling. To do this, line the bottom of the crust with a piece of parchment paper and fill with dried beans or use this handy pie weight. I also line the edges with a pie shield to protect the crust from becoming overly browned. Bake for 15 minutes.
  1 unbaked pie crust shell
• Meanwhile make the custard. In a large bowl whisk together the sugar, flour and salt. Add in the sweetened condensed milk, buttermilk, eggs, lemon juice, and vanilla. Whisk well.
  ⅓ cup granulated sugar, 3 tablespoons all-purpose flour or gluten free flour, ½ teaspoon salt, 14 ounce can sweetened condensed milk, 1 cup buttermilk, 2 large eggs, 1 egg white, 1 tablespoon lemon juice, 1 teaspoon vanilla
• Remove the partially baked bake pie crust from the oven and lower the temperature to 325°F. Pour in the filling, replacing the pie shield after. Bake until the custard is set, about 60-70 minutes.
The top will look slightly puffed out. The edges may start to crack slightly. Press a finger down lightly in the middle. If it feels like liquid underneath, keep cooking. It should be slightly jiggly, but not slosh around and move easily.
• Cool the pie on a wire rack until it comes to room temperature, about 1-2 hours. Meanwhile, make the blueberry topping.

Topping

• In a small saucepan stir together 1 cup blueberries with ¼ cup sugar. Cook over medium heat until blueberries start to burst and juice, about 5 minutes.
  3 cups fresh blueberries, ¼ cup sugar
• Let blueberries simmer, stirring occasionally, until thickened, about 4 minutes. Remove from heat and stir in vanilla.
  
  ½ teaspoon vanilla
• Let blueberry sauce cool down slightly, about 10 minutes. Stir in remaining fresh blueberries. Spread blueberry mixture over top of the cooled pie. Transfer to the refrigerator to chill completely and allow the custard to set before slicing and serving. Store leftovers in refrigerator.