Prep the Oven and Dish: Preheat your oven to 350°F. Lightly grease a glass pie plate or 9-inch baking dish with butter.
Prepare the Fruit: Place the prepared apple slices in a large bowl. Toss thoroughly with fresh lemon juice and ½ teaspoon of ground cinnamon until evenly coated. Transfer the fruit into the greased pie plate, spreading it evenly.
Mix the Dry Topping: In a separate medium bowl, whisk together the white sugar, brown sugar, cinnamon, lemon zest, salt, and flour.
Work in the Butter: Add the softened stick of butter to the flour mixture. Using a large fork or your fingertips, blend the butter into the dry ingredients until the mixture forms loose, pebble-sized crumbles.
Assemble and Bake: Scatter the crumble topping evenly over the fruit in the pie dish, covering the apples completely.
Bake Until Golden: Bake for 50 to 60 minutes, or until the apple filling bubbles, the apples are tender when pierced with a knife, and the streusel topping is golden brown and crispy. Let cool slightly and serve warm.