Place the apple halves onto cutting board with the flat cut side down. Place chop sticks or wooden spoons above and below the apple. Starting at the outermost edges, cut 1/8” thick slices, cutting down to the chopstick or wooden spoon.
Place apple halves cut sides down, in an 8-inch square glass or ceramic baking dish coated with cooking spray. (See Note 3) Combine 1 tablespoon sugar, 1 tablespoon butter, and 1/2 teaspoon cinnamon in a small bowl. Brush mixture evenly over each apple.
Cover with foil and bake at 400°F for 25-30 minutes. Remove foil. and continue baking for another 10 minutes or until apples are tender. Remove pan from oven and allow to cool for 10 minutes.
In a small bowl combine remaining 3 tablespoons sugar, remaining 1 1/2 tablespoons butter, remaining 1/2 teaspoon cinnamon, oats, flour, and salt.
Carefully fan open the apple halve slices and carefully spoon streusel evenly over apples. Bake for 10 minutes. Turn broiler to high (leave pan in oven) and broil apples 2 minutes. Serve with ice cream and optional caramel sauce.
Firm-fleshed apples are needed. Surprisingly, Granny Smith apples do not hold up well. Good options are Honeycrisp, Pink Lady, and Fuji.
Nutritional information shown does not include ice cream or caramel.
The apples can be baked in a cast iron skillet/pan, enameled casserole dish, oven-safe glass baking dish, or on a baking stone. All of these worked well for me.