1. Preheat oven to 375°.
2. To prepare topping, weigh or lightly spoon 1.5 ounces flour into a dry measuring cup; level with a knife. Combine flour and next 3 ingredients (through salt) in a medium bowl, stirring with a whisk; stir in oats. Cut in butter with a pastry blender or 2 knives until crumbly. Cover and chill 20 minutes.
3. To prepare filling, combine 1/4 cup sugar and next 3 ingredients (through salt) in a large bowl, stirring with a whisk. Add apple, and drizzle with lemon juice; toss well. Spoon apple mixture into an 8-inch square glass or ceramic baking dish coated with cooking spray. Sprinkle topping over filling.
4. Bake at 375° for 45 minutes or until apples are tender and topping is crisp. Let stand at least 15 minutes before serving.