Preheat oven to 375°F.

Toss bananas with melted butter, half the brown sugar, and cinnamon in a 9-inch cast iron skillet.

Separately mix oats, flour, and remaining brown sugar; cut in cold butter until crumbly.

Sprinkle crumble evenly over bananas.

Bake 25 minutes, until topping is golden brown and filling bubbles.