Step 1: Preheat Oven and Prepare Pan
Set the oven temperature to 400°F, or 200°C. Line a 9 x 13-inch baking sheet with parchment paper for easy removal, or lightly oil it.
Step 2: Make the Crust and Topping
Put the softened butter, shortening (if using), and brown sugar in a large bowl. Using a medium-speed electric mixer, beat until smooth and creamy. Mix the oats, baking soda, salt, and 1¾ cups flour in a separate basin. Mix the dry ingredients into the creamed mixture until they are crumbly and thoroughly combined.
Step 3: Form the Crust
To make the topping, measure out and save 2 cups of the crumbly mixture. To create a solid crust, evenly press the remaining ingredients into the bottom of the baking pan.
Step 4: Prepare the Apples
To keep the apples from browning, combine the lemon juice and diced apples in a medium bowl. Next, dust the apples with the three tablespoons of flour and toss once more to coat. As the apples bake, the flour helps to thicken their juices. Evenly cover the prepared crust with the apple mixture.
Step 5: Melt the Caramel
Melt the unwrapped caramels with the water or heavy cream in a small saucepan over medium heat, stirring often until smooth. You may omit this step if you are using caramel sauce from a jar.
Step 6: Assemble the Bars
Evenly cover the apple layer with the melted caramel. Cover the caramel and apples with the remaining oat mixture. Using your hands or a spatula, lightly press the topping down.
Step 7: Bake
Bake for 25 to 30 minutes, or until the apples are soft and the topping is golden brown.
Step 8: Cool and Serve
Before slicing the bars into squares, let them cool somewhat. You may either let them cool fully at room temperature or even put them in the refrigerator for a little while if you want cleaner cuts. Any leftovers should be refrigerated, covered.