For crust: I recommend using Dad’s Pie Pastry, but any basic pie pastry will do. Roll the dough into a 13-inch circle and line a 9½ inch deep-dish pie plate with it, forming the overhanging dough into an upstanding rim. Place the pie shell in the freezer for 15 minutes while you make the filling.
For filling: Preheat the oven to 400°F. Combine the peaches and nectarines in a medium bowl. Mix the sugar and cornstarch and stir into the fruit. Stir in the lemon juice and zest. Turn the mixture into the chilled pie shell and smooth the top. Bake on the center oven rack for 15 minutes. While it bakes, make the topping.
For topping: Combine the Heath bar pieces, flour, pecans, sugar, and salt in a food processor. Pulse until the Heath bars are well chopped. Scatter the butter over the mixture. Pulse again until the Heath bars are finely chopped and the crumbs look like gravel. Transfer the topping to a large bowl and rub the mixture between your fingers until the texture is uniform. When the pie has baked for 15 minutes, reduce the heat to 375°F. Remove the pie from the oven and spread the topping evenly over the fruit. Tamp down gently, to even it out. Return the pie to the oven and bake for 40 to 45 minutes more, until the topping is a rich dark brown and any visible juices bubble thickly. Transfer the pie to a rack and cool. Serve slightly warm or at room temperature.