Preheat oven to 375 degrees F. Lightly grease a 9×13-inch baking pan. Bake 30 to 35 minutes until firm.
For the Crust;
1 ½ cup – 2 cups Graham Cracker Crumbs
½ cup melted butter
For the Crumb:
1 cup (packed) dark brown sugar
1/2 cup sugar
1 1/2 tablespoons ground cinnamon
1/2 teaspoon salt
1 cup (2 sticks) unsalted butter, melted, warm
2 1/2 cups all purpose flour
For the Blueberry Filling:
4 cups fresh blueberries
½ cup granulated sugar
4 teaspoons cornstarch
Juice of one lemon
or for Apple Filling;
2 pounds Granny Smith apples, peeled, cored and sliced ¼-inch thick
2 tablespoons butter, melted
¼ cup granulated sugar
½ teaspoon ground cinnamon
1/2 cup Sweetened Condensed Milk
Make the Crust: Melt butter. Press the crumbs into an even layer in the prepared pan.
Make the Crumb: Mix both sugars, cinnamon, and salt in medium bowl and whisk to blend. Add warm melted butter and stir to blend. Add flour and toss with fork until moist clumps form (topping mixture will look slightly wet). Set aside.
For the Filling: In a separate large bowl, whisk together the sugar, cornstarch and lemon juice. Gently fold in the blueberries or apples to coat evenly with the sugar mixture. Spread the blueberry or apple mixture evenly over the crust, drizzling any excess juice in the bowl over the filling. Add the crumb topping evenly over the blueberry or apple layer. Drizzle 1/2 a cup sweetened condensed milk over the dessert. Bake for 30 minutes, or until the top is light golden brown. Cool completely before cutting into squares. Leftovers can be stored in an airtight container or wrapped in plastic wrap at room temperature or in the refrigerator.