- Preheat oven to 375°F.
- Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, sugar, oats, and 1/4 teaspoon cinnamon in a medium bowl; cut in butter with a pastry blender or 2 knives until mixture is crumbly. Stir in walnuts.
- Combine apple and remaining ingredients in a large bowl; toss well.
- Spoon apple mixture into an 8-inch square baking dish or 1 1/2-quart casserole.
- Sprinkle with crumb mixture. Bake at 375°F for 45 minutes or until golden brown. Serve warm.