1. Add berries, sugar and lemon juice to a large saucepan over medium heat
2. Simmer, until warm and juicy, about 5-10 minutes, gently stirring occasionally.
3. Spoon out about 1/2 cup of the juice from the pan into a bowl. Stir cornstarch into the juice until smooth
4. Bring pot of berries back to a simmer and slowly pour in the cornstarch. Gently stir mixture (being careful not to mash the berries), until thickened, about 2-5 minutes.
5. Remove from heat and stir in butter. pour mixture into unbaked pie shell. Add lattice top or a whole top with holes pricked on top for steam to escape.
6. Pinch the edges of the top and bottom pie crusts together and crimp the edge. Brush a thin layer of beaten egg white over the top of the pie and sprinkle lightly with sugar.
7. Bake at 400 degrees F for 40-45 minutes. check it after about 25 minutes and place a piece of tinfoil over it if the top crust is getting too brown.
8. Remove to a wire cooling rack and allow to cool for several ours.