Peel, core and chop the apples; toss in a bowl with lemon juice. In a separate bowl, combine the brown sugar, cinnamon, and nutmeg; add to the apples and toss to combine.
In another bowl combine flour, sugar and oats. Cut butter into 8 small pieces, and cut butter into flour with a pastry blender or two forks until mixture is crumbly. Stir in the chopped nuts.
Butter a 9-inch square baking dish. Spread apple mixture in bottom of baking dish then sprinkle with flour mixture. Bake at 375° for 30 to 45 minutes, or until apples are tender and topping is lightly browned.
Serve warm or at room temperature, with vanilla ice cream or a little heavy cream, if desired.