Whisk flour, brown sugar and apple pie spice in bowl to blend.
Add butter and rub in with finger tips or pastry blender until the size of peas.
Mix in almonds; set aside.
Heat oven to 350 F. Butter 13×9-inch baking dish.
In large bowl, whisk sugar, flour, lemon zest and apple pie spice. Add apple slices and vanilla; toss to coat. Spoon filling into prepared dish. Sprinkle topping over.
Bake until apples are tender and topping is golden and crisp, about 1 hour. Cool at least 10 minutes. Serve warm with ice cream if desired.