To make the crust: Mix together the butter, flour, sugar, and salt until the dough comes together in a ball., Grease a 9" tart pan and press the dough firmly into the base of the pan and up the sides in an even layer. You may find that you have more dough than you need; the crust should be a thin layer, approximately 1/8" thick. Refrigerate the crust while you make the filling., Preheat the oven to 375°F., To make the filling: Beat together the butter and sugar until pale in color and fluffy. Beat in the almond extract. Add the eggs one at a time, beating well after each addition., Whisk together the almond flour and all-purpose flour, then fold into the wet ingredients., Pour the filling into the chilled crust and smooth the top., Arrange the thin apple slices in two concentric circles, overlapping each slice very slightly, over the surface of the tart. Depending on the size of your apples, you may need more or fewer slices., Bake the tart for 40 to 50 minutes, or until the crust is golden brown, the apples begin to brown on the edges, and the filling is puffed and set., Remove the tart from the oven, and let it cool for at least 15 minutes before serving., Store any leftovers in the refrigerator, well wrapped, for several days. Freeze for longer storage.