Preheat oven to 350° Fahrenheit.
For the butterscotch sauce, heat a small saucepan over medium-low heat and melt butter. When completely melted, whisk in dark brown sugar, heavy whipping cream and salt.
Bring mixture to a boil, whisking often and continuing to boil for 7-10 minutes or until sugar and salt dissolve, the mixture begins to thicken and develops a nutty caramel aroma. (Make sure not to burn.) Remove from heat and stir in vanilla. Set aside.
For the filling, in a large mixing bowl, stir together apples, lemon juice, sugar, cinnamon and cornstarch..
Prepare an 8 x 8 or 9 x 9 inch baking dish by coating lightly with butter.
Place apple slices into prepared pan and spread them out evenly. Pour butterscotch topping over the apple filling, reserving ½-1 cup for serving. Set aside.
Use a separate mixing bowl to prepare the topping. Combine rolled oats, flour, cinnamon, nutmeg, all-spice and salt. Mix well.
Pour melted butter into oat mixture and use a fork to stir the mixture until it is well-combined and crumbly.
Sprinkle topping mixture over the apple filling
Bake for 45-50 minutes or until topping is golden brown and filling is warmed through and bubbling.
Serve warm with a scoop of vanilla ice cream and a drizzle of the reserved butterscotch sauce.