Preheat oven to 190C (375F).
Butter 25x25 cm baking dish (approx. 9x9 inch) or similar size.
In a bowl, combine rolled oats, flour, brown sugar, cinnamon, baking powder, and salt. Using your fingertips rub the butter into dry ingredients until crumbly. Transfer to refrigerator to chill.
For the filling, in a large bowl whisk together melted butter, cornstarch, water, lemon zest, brown sugar, and cinnamon. Add in sliced plums and apples, and toss everything together so that fruit is evenly coated with butter mixture.
Place apples and plums into the prepared baking dish and sprinkle with chilled topping.
Bake until the top is golden brown and fruit is tender around 30-35 minutes.
Serve warm with vanilla ice cream.