Heat the oven to 375 degrees F. Butter a 13-by-9-inch baking dish. Any 4-quart baking dish works.

Core and cut the apple into 4 quarters, slice them into about 1/4 inch slices. Add 1 tablespoon lemon (juice of half lemon). It helps the apple from browning.

Heat a pan over medium heat, add 2 tbsp butter, when it melts add 1/4 cup brown sugar, 1/4 sugar white sugar, 1 tbsp water, 1/4 tsp salt, and sliced apples. When the apple softens, transfer this to the baking dish. Keep it aside.

In a medium bowl, whisk the 1/2 cup flour, 1/4 cup brown sugar, 1/4 cup white sugar, 1/2 tsp cinnamon, and 1/4 tsp salt. Add the cubed butter, and stir with a fork or pastry cutter until the crumbs are evenly moistened. Stir in the rolled oats.

Add 1 tablespoon corn starch or all-purpose flour to the filling and mix it very well.

Sprinkle the toppings over the apple and do not press it down

Bake at 350 degrees F for about 25 - 30 minutes or until the topping is golden brown and the juices are bubbling around the edges.

Remove the pan from the oven and place it on a wire rack to cool for at least 5 minutes before serving. Serve it plain or with vanilla ice cream, warm, room temperature.

You can store the leftover (covered) in the refrigerator for up to 5 days.