Preheat oven to 350 degrees F.
On a lightly floured surface, roll out half of dough for bottom crust. Fit into an 11-inch pie plate. Trim pastry using scissors or a sharp paring knife and chill. Keep remaining one-half of dough refrigerated wrapped in plastic for lattice.
Combine rhubarb and raspberries in a large bowl. Sprinkle with sugar, 1/3 cup flour, and lemon juice. Toss gently so as not to break raspberries. Spoon into pie shell and dot with butter. Refrigerate while preparing lattice top.
In a small bowl combine egg and heavy cream, whisk lightly to combine; set aside. On a lightly floured surface roll out remaining piece of dough for lattice and cut it into 1-inch strips. Weave strips together over filling, and place an extra strip or two along the edge, where bottom crust joins lattice top; press this strip with tines of a fork to seal well. Brush lattice and all pastry edges with egg glaze, sprinkle top with sugar and place pie on a Silpat or parchment-lined baking sheet.
Bake until pastry is golden brown and juices in center of pie are bubbling, about 1 1/4 to 1 1/2 hours. Transfer to a wire rack to cool before serving.
3 3/4 cups all-purpose flour
1 1/2 teaspoons salt
1 1/2 teaspoons sugar, optional
1 1/2 cups (3 sticks) cold unsalted butter, cut into small pieces
6 to 12 tablespoons ice water
In the bowl of a food processor, add flour, salt, and sugar. All ingredients should be cold. Add the pieces of butter and process for approximately 10 seconds, or just until the mixture resembles a coarse meal. (To mix by hand, combine the dry ingredients in a large mixing bowl. Using a pastry blender or 2 table knives, cut in the butter until the mixture resembles coarse meal.)
Add ice water, drop by drop, through feed tube with machine running, just until dough holds together without being wet or sticky; do not process more than 30 seconds. Test dough at this point by squeezing a small amount together. If it is crumbly, add a bit more water.
Divide dough in half. Place 1 half on a piece of plastic wrap. Grasping ends of the plastic wrap with your hands, press the dough into a flat circle with your fists. This makes rolling easier than if the pastry is chilled as a ball. Repeat with second half. Wrap the dough in the plastic and chill for at least 1 hour.
Yield: enough for 1 (11-inch) double-crusted pie.
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