To make the crust, mix the butter and sugar together until light and fluffy. Add the flours and salt until a soft dough forms. Press the dough into a 13×9 pan that has been sprayed withcooking spray. Bake in a 375˚F oven for about 20 minutes or until the crust is golden brown. Set aside to cool.
Melt the butter and brown sugar in a large skillet over medium high heat. Add the apples to the skillet and sauté for 10 minutes or until the apples have softened. Stir in the cinnamon and nutmeg and continue sautéing for about 8 more minutes or until the apples have slightly caramelized. Add water to the skillet as necessary if the apples seem overly dry and/or to prevent scorching. Remove from heat and set aside to cool.
In a large bowl, mix the oats, flours, brown sugar, cinnamon, baking soda and salt. Add the butter and mix with a pastry cutter, two forks or your fingers until the mixture resembles tiny pebbles. Set aside.
Scatter apples over the top of the crust and then top with the crumb mixture. Bake in a 375˚F oven for 35-40 minutes or until the topping is golden brown. Serve bars warm with a drizzle of caramel sauce and a scoop of vanilla ice cream. Bars will keep in a covered container at room temperature for up to three days.