To prepare crust: Whisk whole-wheat flour, all-purpose flour, 2 tablespoons sugar and salt in a large bowl. Cut butter into small pieces and, with your fingers, quickly rub them into the dry ingredients until smaller but still visible. Add sour cream and oil; toss with a fork to combine with the dry ingredients. Sprinkle water over the mixture. Toss with a fork until evenly moist. Knead the dough with your hands in the bowl a few times"the mixture will still be a little crumbly. Turn out onto a clean surface and knead a few more times, until the dough just holds together. Divide the dough in half and shape into 5-inch-wide disks. Wrap each in plastic and refrigerate for at least 1 hour.
To prepare filling: Just before you’re ready to roll out the dough, process tapioca in a spice grinder, mini food processor or blender until finely ground. Combine with strawberries, rhubarb, sugar, lemon juice, nutmeg and salt in a large bowl; toss well to combine.
Position a rack in the center of the oven and place a foil-lined baking sheet on the rack below; preheat to 425°F.
Remove the dough from the refrigerator; let stand for 5 minutes to warm slightly. Roll one portion between sheets of parchment or wax paper into a 12-inch circle. Peel off the top sheet and invert the dough into a 9-inch pie pan. Peel off the remaining paper. Moisten the outer edge of the dough with water. Scrape the filling and any accumulated juices into the crust.
To prepare lattice top: Roll the remaining dough between sheets of parchment or wax paper into a 12-inch circle. Peel off the top sheet. Cut the dough into 1-inch strips using a pastry wheel or a knife. Lift off every other strip and lay them on top of the pie, leaving about a 1-inch gap between strips. Use the shorter strips for the edges and the longer ones for the middle of the pie. (You may not need to use the outermost strips.) Fold back the first, third and fifth strips of dough to the edge of the pie. Place a shorter strip of dough across the second and fourth strips, about 1 inch from the edge. Unfold the folded strips over the crosswise strip. Fold back the second and fourth strips over the first crosswise strip. Place another strip crosswise, about 1 inch from the first. Unfold the strips over the second crosswise strip. Continue folding back alternating strips and placing crosswise strips until the top is covered with woven strips. Trim any overhanging crust. Crimp the outer edge with a fork. Brush the dough with egg white; sprinkle 1 teaspoon sugar (if using) over just the lattice top, not the outer edge.
Bake the pie for 20 minutes. Then rotate the pie 180 degrees and lower the oven temperature to 325°. Continue baking until the crust is golden and the filling is beginning to bubble, 30 to 35 minutes more. Let cool on a wire rack for at least 2 hours before serving.
Per serving :
10 g Fat;
4 g Sat;
3 g Mono;
14 mg Cholesterol;
47 g Carbohydrates;
4 g Protein;
3 g Fiber;
82 mg Sodium;
169 mg Potassium
Make Ahead Tip: Refrigerate dough for up to 2 days or freeze for up to 6 months. | Equipment: 9-inch pie pan
Note: Whole-wheat pastry flour contains less gluten than regular whole-wheat flour and helps ensure a tender result in delicate baked goods. Find it in the baking section of the supermarket or online at bobsredmill.com and and kingarthurflour.com.