Preheat the oven to 350°F
For the butterscotch sauce, melt butter in a small saucepan over medium-low heat. Once melted, whisk in dark brown sugar, heavy cream, and salt.
1/2 cup (113 g) unsalted butter, 1 cup (205 g) packed dark brown sugar, 1 cup (236 ml) heavy whipping cream, 1/2 tsp kosher salt
Bring the mixture to a boil, whisking often. Continue boiling for 7-10 minutes, or until the sugar and salt dissolve, the sauce thickens, and a nutty caramel aroma develops. (Be careful not to burn it.) Remove from heat, stir in the vanilla, and set aside.
1-1/2 tsps vanilla extract
For the filling, stir together apples, lemon juice, sugar, cinnamon, and cornstarch in a large mixing bowl.
5-6 Granny Smith apples, cored, peeled and thinly sliced, 2 tbsps fresh lemon juice , 2 tbsps granulated sugar, 1 tsp ground cinnamon, 2 tbsps cornstarch
Lightly coat an 8x8 or 9x9-inch baking dish with butter.
Place the apple slices in the prepared pan and spread them out evenly. Pour the butterscotch topping over the apples, reserving ½ to 1 cup for serving. Set aside.
In a separate mixing bowl, combine rolled oats, flour, cinnamon, nutmeg, allspice, and salt. Mix well.
1 cup (108 g) old-fashioned oats, 1 cup (120 g) all-purpose flour, 1 tsp ground cinnamon, 1/2 tsp ground nutmeg, 1/2 tsp ground allspice, 1/2 tsp kosher salt
Pour the melted butter into the oat mixture and use a fork to stir until well combined and crumbly
1/2 cup unsalted butter, melted
Sprinkle the topping over the apple filling. Bake for 45-50 minutes, or until the topping is golden brown and the filling is bubbling and warmed through.
Serve warm with a scoop of vanilla ice cream and a drizzle of the reserved butterscotch sauce.
vanilla ice cream for serving (optional)