Prepare the pie pastry; refrigerate until ready to use.

In a large mixing bowl, combine the prepared and chopped apples and rhubarb and toss with sugar, flour, and cinnamon, and water to coat; set aside (the mixture will be crumbly).

On a well-floured surface, roll 1/2 of the pastry out into a large circle, about 1/8" thick, moving the pastry around as you go and adding more flour to prevent sticking.

Fold the pastry in half, and lift it into a ungreased 9" or 10" pie plate, to cover 1/2. Unfold the pastry to cover the rest of the pie plate; trim the excess with a knife, and set the scraps aside.

Note: Folding the pastry before you move it is to prevent the dough from tearing.

Stir the fruit mixture again, just to distribute the apple juices a little bit; pour into prepared pie crust. Dot with butter, and set aside.

On a well-floured surface, roll the remaining 1/2 of the pastry out into a large circle. Cut into 1/2" strips.

Using a pastry brush or your fingertips, moisten the edges of the pie crust in the plate with water, just until lightly damp. Layer strips of the cut pastry on top, starting with the largest piece in the middle of the pie, and pressing the edges lightly to attach to the rim. Lay the pieces on in an"X" pattern; start with a large piece right down the middle of the pie, followed with another long piece in the opposite direction. Layer each piece 1/2" apart from each other, rotating which side you lay the pieces down, until the whole pie is covered. Trim the excess, and set aside the scraps.

Flute the edges or press with a fork to seal the lattice strips and base pie crust together.

Brush the lattice strips with beaten egg, and sprinkle with coarse sugar.

Preheat the oven to 450 degrees F. When the oven is hot, bake the pie for 15 minutes, reduce the temperature to 350 degrees F, and bake another 25-30 minutes, or until the crust is golden brown and the filling is bubbly. Cool completely before serving.