1Preheat oven to 375 degrees F (190 degrees C). Spray a 9x13-inch baking dish with cooking spray.
2Mix together 1 cup of rolled oats, 1 tablespoon of butter, and 2 tablespoons of brown sugar in a bowl; press the mixture into the bottom of the prepared baking dish. Lightly toss together the plums, white sugar, flour, cinnamon, and nutmeg in another bowl. Spread the plum mixture over the crust. Mix 1 cup of oats, 2 tablespoons of brown sugar, and 1 tablespoon of butter in a bowl until well combined, and sprinkle the crumble mixture over the top of the plums.
3Bake in the preheated oven until the plum filling is bubbling and the crisp topping is browned, about 35 minutes. Serve warm.