3 large firm, crisp-sweet apples, such as Pink Lady or Honeycrisp (do not use a soft variety like Cortland or McIntosh—they will not hold their shape)
3 tablespoons unsalted butter, divided
3 1/2 tablespoons brown sugar, divided
1 1/2 teaspoons ground cinnamon, divided
3 tablespoons old-fashioned rolled oats
2 teaspoons white whole wheat flour (or all purpose flour)
1/4 teaspoon kosher salt
Low-fat vanilla ice cream or vanilla frozen yogurt, for serving
Place a rack in the center of your oven and preheat the oven to 400 degrees F. Coat the inside of a 9×13-inch or similar glass or ceramic baking dish with nonstick spray and set aside.
Peel the apples, then cut them in half from top to bottom. With a melon baller or small spoon, remove the core. Turn the apples cut-side down on a cutting board. Starting at the outermost edges, cut 1/4-inch slices most (but not all) of the way through each apple. Place apple halves, cut sides down, in the prepared baking dish.
Melt 1 1/2 tablespoons butter in a small saucepan or microwave-safe bowl. Stir in 1 1/2 tablespoons brown sugar and 1/2 teaspoon cinnamon. Brush the mixture evenly over each apple.
Cover the pan with foil. Bake for 20 minutes, then remove the pan from oven, uncover, and let cool 10 minutes. Increase the oven temperature to 425 degrees F.
While the apples cool, make the streusel topping: Melt the remaining 1 1/2 tablespoons butter in the same bowl or saucepan used previously. Stir in the remaining 2 tablespoons brown sugar, 1 teaspoon cinnamon, oats, flour, and salt. Carefully fan open the apple halves and with a fork or spoon. Work the some streusel between the slices and sprinkle some over the top.
Return to the pan to the oven and bake uncovered at 425 degrees F until the apples are tender and the topping is brown, about 10 to 15 additional minutes. Let cool for a few minutes, then serve warm with ice cream.