This is from the PBS show, America's Test Kitchen. They exhaustively test recipes to develop the best method and ingredients to achieve a given result.

This blueberry pie recipe is meant to come out juicy but hold its shape.

6 cups fresh blueberries
3/4 cup sugar
2 tablespoons Minute tapioca, ground finer in a coffee grinder
2 teaspoons lemon juice
2 teaspoon lemon zest
Pinch salt
1 granny smith apple
2 tablespoons butter
1 egg beaten with 1 teaspoon water
Pastry for a double-crust 9 inch pie

Put 3 cups berries into a saucepan and mash with a potato masher while cooking over medium heat to a jam like consistency, about 1 1/2 cups, for about 8 minutes. Cool slightly, put into mixing bowl and add remaining berries. Add the sugar, tapioca, lemon juice, zest, and salt. Peel the apple and grate it. Put onto a tea towel and squeeze out the moisture. Combine with the berries and pour into pie crust. Dot with the 2 tablespoons butter. Cut several holes in the top crust with a small biscuit cutter to mimic a lattice top. Place on pie, and trim both crusts to a 1 inch border all around. Tuck both crusts under all the way around, and crimp. Brush with the egg wash. Bake at 400 degrees for 30 minutes, then reduce to 350 degrees for 30 more minutes. Put a sheet pan on the rack under it, preheat, to help bake the bottom crust. Cool to room temperature before cutting.