Preheat the oven to 400°F. Generously spray an 8x8-inch baking dish. Set it aside.
Using a cutting board and a sharp knife, place the halved apple cored side down. Starting on the outside of the apple, carefully cut slits almost, but not completely, through the apple. Space the slices ⅛ inches down the apple. Repeat for the remaining apple halves.
Place the slitted apple halves, cored side down, into the prepared baking dish.
Using a small mixing bowl, combine 2 ½ tablespoons of brown sugar, 2 tablespoons of melted butter, and ½ teaspoon ground cinnamon.
Using a pastry brush, brush the sugar mixture evenly over the 4 apple halves.
Cover the baking dish with aluminum foil, and bake for 20 minutes.
Remove the baking dish from the oven, and allow the covered baking dish to rest for 5 minutes.
Using a small mixing bowl, combine the remaining brown sugar, melted butter, and ground cinnamon. Add the pecans and flour. Mix until smooth.
Remove the foil from the baking dish. Using 2 forks, carefully fan out the slices in the apple.
Spoon the pecan mixture over the halved apples.
Return the uncovered baking dish to the oven, and bake for a further 15 minutes.
Remove the baking dish from the oven, and allow the apples to rest for 5 minutes. Serve with a scoop of vanilla ice cream and a drizzle of caramel sundae topping.