Weigh or lightly spoon flour into dry measuring cups; level with a knife.
Combine flour, brown sugar, oats, 1 teaspoon cinnamon, salt, and, if desired, nutmeg, in a bowl.
Add butter; beat with a mixer at low speed 3 minutes or until moist and crumbly.
Place apple in a large bowl; sprinkle with lemon rind and lemon juice, and toss well.
Sprinkle with granulated sugar and remaining 1 teaspoon cinnamon; toss well. Spoon apple mixture into an 8-inch square glass or ceramic baking dish coated with cooking spray.
Sprinkle brown sugar mixture over apple mixture.
Bake, uncovered, at 400 for 15 minutes. Reduce oven temperature to 35
Bake an additional 40 to 50 minutes or until apples are tender and topping is crisp and brown.