1. Preheat oven to 400°.
2. 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.
3. 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 350°.
4. Bake an additional 40 to 50 minutes or until apples are tender and topping is crisp and brown. Let stand 15 minutes.