Add berries, sugar and lemon juice to a large saucepan over medium heat. Simmer, until warm and juicy, about 5-10 minutes, gently stirring occasionally. (You can taste it at this point and see if it's sweet enough for your liking. I don't like to make mine too sweet, but if you want it sweeter you can add ¼ cup more sugar.)

Spoon out about ½ cup of the juice from the pan into a bowl. Stir cornstarch into the juice until smooth. 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.

Remove from heat and stir in the butter. Allow to cool for 15 minutes. Pour mixture into unbaked pie shell (in a 9 in deep dish pan). Add lattice top or a whole top with holes pricked on top for steam to escape.

Pinch the edges of the top and bottom pie crusts together and crimp the edge, if you like. Brush a thin layer of beaten egg white over the top of the pie and sprinkle lightly with sugar. 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. Remove to a wire cooling rack and allow to cool for several hours.