Heat the oven: Preheat to 350°F (175°C). Lightly butter or spray a 9-inch square baking dish or similar.

Prep the bananas: Peel and slice bananas into thick rounds. Toss with lemon juice, 1 tablespoon brown sugar, 1/2 teaspoon cinnamon, vanilla, and a pinch of salt. Spread evenly in the baking dish.

Make the crumble: In a bowl, combine oats, flour, remaining 2 tablespoons brown sugar, granulated sugar, remaining 1/2 teaspoon cinnamon, nutmeg (if using), and a pinch of salt. Add cold butter cubes.

Work in the butter: Use your fingers or a pastry cutter to rub butter into the dry mix until it forms pea-sized clumps. Stir in nuts if using. The mixture should be crumbly, not pasty.

Top the bananas: Sprinkle the crumble evenly over the banana layer. Don’t press it down; you want craggy bits for extra crunch.

Bake: Bake for 25–30 minutes, until the top is golden and the edges are bubbling.

Rest and serve: Let it cool for 10 minutes so the juices thicken slightly. Serve warm, ideally with a scoop of vanilla ice cream or a dollop of whipped cream.