How to Make It
Preheat oven to 400°F.
Starting at the outermost edges, cut most (but not all) of the way through each apple half at 1/8-inch intervals. Place apple halves, cut sides down, in an 8-inch square glass or ceramic baking dish coated with cooking spray. Combine 1 tablespoon sugar, 1 tablespoon butter, and 1/2 teaspoon cinnamon; brush mixture evenly over apple.
Cover pan with foil; bake at 400° for 20 minutes. Remove foil. Bake at 400° for 10 minutes or until apples are tender. Remove pan from oven; cool 10 minutes.
Combine remaining 3 tablespoons sugar, remaining 1 1/2 tablespoons butter, remaining ¼ teaspoon cinnamon, oats, flour, and salt. Carefully fan open apple halves. Spoon oat mixture evenly over apples. Bake at 400° for 10 minutes. Turn broiler to high (leave pan in oven); broil 2 minutes. Serve with ice cream.
Make Ahead Tip: Cook apples in advance, but hold off on step 4. Cool and refrigerate baked apples until ready to serve; then top with streusel and bake as directed.