Prepare the pie crust and shape half of the dough into a pie plate. Slice the other half into strips and cover both with plastic wrap and place in the fridge.
Preheat the oven to 375 degrees.
Add the fruit, sugar and pectin into a saucepan and gently, using a rubber spatula, fold the berries in. Start with only 1/2 cup sugar, but if your berries are extra tart you should add more sugar, 1 tablespoon at a time. Turn the heat to medium low and cook until the berries are releasing their juices and beginning to thicken. Only stir occasionally as you don't want to break up the fruit and turn it into jam.
Add the hot fruit mixture directly into the pie crust. Place strips across the pie and then lift up one end of every other strip and lay a new strip across the other way. Now, repeat but with the other strips that were not originally lifted up. Continue until you're halfway across the pie then turn the pie around and repeat from the other side to make a lattice across the entire pie.
Whisk together the egg and water and then brush the top of the pie and immediately sprinkle with sugar.
Bake the pie for 25 minutes then drape with a piece of tin foil and continue to bake for another 20-30 minutes, or until the filling is bubbly and the crust is golden brown.
Remove from the oven and cool entirely on the counter. We prefer to let our pie rest over night as the filling will set up and not run. Refrigeration can make the filling sink and pull away from the crust.