How to Make It
In a food processor, pulse the 2 cups of flour with the salt. Add the butter and pulse until the pieces are the size of small peas. Sprinkle the water on top and pulse until the dough just comes together. Turn the dough out onto a work surface, gather up any crumbs and pat into a disk. Wrap in plastic and refrigerate until well chilled, 1 hour.
Preheat the oven to 400°. Spread the walnuts in a pie plate and bake for about 8 minutes, until lightly browned. Let cool, then chop.
In a medium bowl, whisk the flour with the brown sugar and salt. Add the butter and, using your fingers, pinch it into the dry ingredients until the mixture resembles coarse meal. Add the walnuts and pinch the streusel into clumps. Refrigerate until chilled, about 15 minutes.
Line a rimmed baking sheet with parchment paper. In a large bowl, toss the apples with the pears, 1/4 cup of granulated sugar, the salt and lemon juice. On a lightly floured work surface, roll out the dough to a 19-by-13-inch oval. Ease the dough onto the prepared baking sheet. Mound the filling in the center of the oval, leaving a 2-inch border. Sprinkle the streusel evenly over the fruit and fold the edge of the dough up and over the filling.
Brush the crust with the egg wash and sprinkle evenly with granulated sugar. Bake the galette for 45 to 50 minutes, until the fruit is tender and the streusel and crust are golden brown. Let the galette cool. Dust with confectioners’ sugar, if using, before serving.