Preheat the Oven: Preheat your oven to 375°F (190°C).1
Prepare the Banana Filling: In your baking dish, arrange the sliced bananas in a single layer. Sprinkle with brown sugar, lemon juice (if using), and cinnamon (if using). Gently toss to combine.
Make the Crumble Topping: In a mixing bowl, whisk together the flour, rolled oats (if using), granulated sugar, brown sugar, and cinnamon (if using). Add the cold, cubed butter. Use a fork or a pastry blender to cut the butter into the dry ingredients until the mixture resembles coarse crumbs. You can also use your fingertips to rub the butter in.
Assemble and Bake: Sprinkle the crumble topping evenly over the banana mixture in the baking dish.
Bake: Bake in the preheated oven for 30-40 minutes, or until the topping is golden brown and the banana filling is bubbly.
Cool and Serve: Let the banana crumble cool slightly before serving. It's best served warm, with ice cream, whipped cream, or custard, if desired.