Make the crust: Cook the bacon in a large skillet over medium heat, turning occasionally, until golden but still soft, about 6 minutes; remove to paper towels. Reserve 1 tablespoon of the drippings in a small bowl; refrigerate until cold, about 15 minutes.
Pulse 2 slices of the bacon in a food processor until finely ground. Add 1 1/2 cups flour and 2 tablespoons sugar and pulse to combine. Add the chilled bacon drippings and pulse to combine. Add the butter and pulse until the mixture looks like coarse meal with some pea-size pieces. Drizzle in 4 tablespoons cold water and pulse until the dough just comes together. Turn out onto a piece of plastic wrap; use the plastic to help form the dough into a disk. Wrap and chill until firm, at least 1 hour.
Line a baking sheet with parchment paper. Roll out the dough into a 13-inch round between 2 sheets of lightly floured parchment. Peel off the top sheet of parchment, then invert the dough onto the prepared baking sheet. Peel off the sheet of parchment. Refrigerate the dough until ready to use.
Preheat the oven to 375 degrees F. Make the filling: Peel the apples and slice 1/4 inch thick. Toss with the lemon juice, cinnamon, 3 tablespoons sugar and the remaining 1 tablespoon flour in a large bowl. Arrange the apples in the center of the dough, leaving a 2-inch border. Fold the edge of the dough over the filling, pleating as needed. Cut the remaining 4 slices bacon into bite-size pieces and sprinkle over the filling.
Brush the crust with the egg and sprinkle the remaining 3 tablespoons sugar over the whole galette. Bake until the apples are tender and the crust is golden brown, about 50 minutes. (Tent loosely with foil if the filling gets too brown.) Remove from the oven and brush the filling with maple syrup while still hot. Transfer to a rack to cool slightly before serving.