Peel, core, and dice the apples into roughly 1/2-inch chunks using a peeler and corer.
Place the apple chunks in a bowl, then toss with lemon juice, sugar, and cinnamon until evenly coated. Set aside to let the flavors meld.
In a separate bowl, combine rolled oats, flour, brown sugar, and a pinch of salt if desired. Mix well.
Add the cold, cubed butter to the dry ingredients. Use your fingers or a pastry cutter to blend the butter into the mixture until it resembles coarse crumbs with pea-sized pieces. This creates a flaky, tender crumble topping.
Press half of the crumble mixture firmly into the bottom of a greased 9-inch square baking pan to create an even base layer.
Spread the prepared apple filling evenly over the crust layer, making sure to distribute the chunks uniformly.
Sprinkle the remaining crumble mixture over the apples, pressing down gently to help it adhere as it bakes.
Bake in a preheated oven at 180°C (350°F) for approximately 35-40 minutes, until the topping turns golden brown and fragrant, and the filling is bubbling around the edges.
Remove from the oven and allow the bars to cool in the pan for at least 15 minutes. This helps them set and makes slicing easier.
Slice into squares or rectangles with a sharp knife, wiping the blade between cuts for clean edges.
Serve warm or at room temperature, optionally with whipped cream or a drizzle of caramel for extra indulgence.
