Make the dough: In a large bowl, whisk the flours with the salt. Using a pastry cutter, cut the butter and shortening into the flour mixture until the pieces pea-sized and coated well in flour. Some large pieces of butter and shortening are good!

Add ice water a little at a time and mix with your hands until the dough just comes together. Pat dough into a large disk and wrap in plastic wrap. Refrigerate for 45 mins.

Make the streusel: Preheat the oven to 400°F. Toast the walnuts in a single layer for about 7 minutes. When cooled, chop into small pieces. In a small bowl, combine the flour with the sugar and salt. Using a pastry cutter, cut the butter into the dry ingredients until the mixture is coarse crumbs. Add the chopped walnuts and oats. Ball the streusel into dime-sized clumps. Chill dough in freezer for 20 minutes.

Make the filing: Line a rimmed baking sheet with parchment paper. In a large bowl, toss the apples with the pears, brown sugar, salt, cinnamon and lemon juice. On a floured work surface, roll out the dough to an 18- x 12-inch inch oval. Carefully lift the dough onto the baking sheet. Pile the filling in the center of the oval (or arrange nicely), leaving about a 2-inch border around all sides. Sprinkle the streusel over the fruit and fold the edge of the dough up and around it, folding the edges as you go along.

Brush the crust with the milk and sprinkle brown sugar. Bake for 50 to 55 minutes, until the fruit is soft and the crust is golden. Let the pastry cool completely before cutting. Keep at room temperature for 3 days or keep refrigerated for up to 5 days.