Preheat oven to 350 degrees F. Liberally spray three 12-cavity muffin tins with cooking spray; set aside.
First, make your apple filling: In a medium saucepan, combine the finely diced apples, butter, brown sugar, granulated sugar, cinnamon and cornstarch together over medium-heat. Cook and stir until bubbling and thickened and apples are tender, about 5-7 minutes. Cool for about 10 minutes.
Unroll one pie crust at a time and use a 2 & 1/2" biscuit cutter to cut circles from the pie crust. You should get about 12 circles per pie crust, so 24 in total. Reroll the pie crusts and cut out additional circles of dough for a total of 36 circles.
Press each circle into the bottom and slightly up the sides of each muffin cavity. Top with a heaping Tablespoonful of the apple mixture.
For the crumb topping: Mix together the flour, brown sugar, granulated sugar and oats in a medium bowl. Pour in the melted butter and toss together until moistened. Drop 2-3 Tablespoonfuls of the crumb mixture on top of the apples in each muffin cavity, gently pressing the crumb mixture down into the cup to compact it some.
Bake for 18-22 minutes or until the tops are light golden brown and the centers are bubbly. Remove from the oven and allow the cookies to cool in the muffin tins for about 15-20 minutes before removing. To remove, gently run the tip of a plastic knife around the edges and slightly under the bottoms of the cookies. They should pop right out.