Preheat your oven to 350°F (175°C). This ensures that your crumble bakes evenly and the apples cook through.

In a mixing bowl, combine the sliced apples with lemon juice, granulated sugar, ground cinnamon, and vanilla extract. Toss gently to coat the apples evenly. The lemon juice helps to keep the apples from browning and adds a nice tang.

Transfer the apple mixture to a 9-inch baking dish, spreading it out evenly.

In a separate mixing bowl, combine the all-purpose flour, old-fashioned oats, brown sugar, and salt. Mix well to combine.

Add the cold, cubed butter to the flour mixture. Using your fingers or a pastry cutter, work the butter into the dry ingredients until the mixture resembles coarse crumbs. This step is crucial as it creates the crumble texture.

Sprinkle the crumble topping evenly over the apple filling in the baking dish.

Place the baking dish in the preheated oven and bake for 30-35 minutes, or until the topping is golden brown and the apples are bubbling. This allows the flavors to meld and the topping to crisp up.

Once baked, remove from the oven and let it cool for about 10 minutes before serving. This helps the juices to settle.

Serve warm, optionally with a scoop of vanilla ice cream or a dollop of whipped cream for a delightful contrast.