1
: Filling: Melt butter in large pan over low heat. Add sugar, cinnamon, ginger, nutmeg and apples. Cook, stirring occasionally, until apples have softened. Stir in cornstarch and cook 30 seconds longer. Remove pan from heat.
2
: Crust: Heat oven to 400ºF (200ºC). Pulse flour, oats, white and brown sugars, baking soda and salt together in food processor. Add butter and egg yolk; pulse until mixture resembles coarse meal. Reserve 1 1/2 cups (375 mL) for topping. Press remaining filling firmly into non-stick 13 x 9-inch (3.5 L) pan. Bake crust 7 minutes. Remove pan from oven.
3
: Assemble and Bake: Reduce heat to 350ºF (180ºC). Gently brush crust with melted butter. Sprinkle oats over crust. Top with apple filling. Pour sweetened condensed milk or caramel flavoured sauce evenly over apples. Top with reserved crust mixture. Bake until filling is bubbling and topping is lightly golden, about 25 minutes. Cool in pan on wire rack. Refrigerate to set, about 2-3 hours. Cut into squares. May be served chilled, at room temperature or warmed.