Peel, core, and chop the apples into small cubes, then toss them with lemon juice, sugar, and cinnamon until evenly coated. Spread the apple mixture in an even layer in the prepared baking pan.
In a large mixing bowl, combine flour, rolled oats, brown sugar, cinnamon, and ginger if using. Mix well to distribute the spices.
Add the cold, diced butter to the dry ingredients. Use your fingertips or a pastry cutter to cut the butter into the mixture until it resembles coarse crumbs, with some larger chunks for texture.
Sprinkle the crumble topping evenly over the apple filling, covering all the fruit surface to create a thick, crumbly layer.
Bake in the preheated oven at 180°C (350°F) for 35-40 minutes, until the topping is golden brown and bubbling around the edges. The aroma of spices and baked apples will fill your kitchen.
Remove from the oven and let the bars cool on a wire rack for at least 20 minutes, allowing the filling to set and making slicing easier.
Cut the cooled bars into squares or rectangles. Serve warm or at room temperature, perhaps with a dollop of whipped cream or a scoop of vanilla ice cream for extra indulgence.
