Prepare Dough: Mix flour, sugar, and salt. Cut in cold butter until crumbly. Add ice water gradually until dough forms. Divide, wrap, and chill for 30 minutes.
Preheat Oven: Set oven to 375°F (190°C). Place a baking sheet in the oven to preheat.
Prepare Filling: Toss sliced apples, pears, sugar, cinnamon, nutmeg, lemon juice, and flour. Let sit for 15 minutes to release juices.
Roll Out Dough: Roll out one dough disk on a floured surface to fit pie dish. Transfer and press into the bottom of the dish. Roll out the other disk and cut into strips for lattice.
Assemble Pie: Fill the crust with the apple-pear mixture. Create a lattice pattern with the dough strips over the filling. Trim and crimp edges.
Brush and Bake: Brush the lattice with egg wash and sprinkle with sugar. Place the pie on the preheated baking sheet. Bake for 45-50 minutes until golden and bubbly.
Cool and Serve: Allow the pie to cool on a wire rack for at least 2 hours before slicing. Serve with ice cream or whipped cream, if desired. Enjoy your delicious Apple-Pear Lattice Pie!