Combine the sugar, salt, and cornstarch in a small bowl. Mix well.
Add 4 cups of frozen berries and lemon zest to the pan. Pour the sugar mixture over this and mix well. Reserve the other 1 cup for later.
Turn heat on medium and cook, and add in the water. Make sure you are stirring continuously so it doesn't burn. It will melt and start to cook and thicken.
Once it has cooked for several minutes and is thickening, add in the remaining cup of berries. Continue to cook for another 2-3 minutes. Remember to stir, with this much cornstarch, it will burn easily.
Remove it from the heat and let it cool.
While it is cooling, prepare your pie crusts. Roll out the bottom and fit it into your pan. Fold over the edges and crimp.
Decide how you want to do the top crust.
Once the filling is cooled to room temp, fill the bottom crust and top as you like.
If you are using a top crust, make sure to brush it with egg wash and sprinkle it with sugar.
Place in the oven and bake until it is golden brown. You may need to add a pie shield after about 20-25 minutes. My pie took about 40 minutes to bake. Just keep an eye on it.
Remove it from the oven and let it cool. Then, place it in the oven. If it is at room temperature, it won't hold as well and will be runnier than if it is chilled overnight in the fridge.