Preheat oven to 350ºF. Lightly butter a 10-inch baking dish or pie plate.
In a medium-size bowl, combine apples with lemon juice and zest. Sprinkle the sugar, cinnamon and flour over the apples and stir to combine. Spoon into the baking dish.
For the crumble topping, combine the dry ingredients in a medium-size bowl. Stir in the melted butter. Spread the topping evenly over the apples.
Place baking dish on a parchment- or foil-lined baking sheet; bake until the juices are bubbling and thick, 40 to 50 minutes. Serve warm with whipped cream.