1) Peel, core, and cut the pears and apples into large chunks. Place the fruit in a large bowl and add the zests, juices, sugar, flour, cinnamon, and nutmeg. Pour into a 23cm x 30cm x 5cm oval baking dish.
2) For the topping: Combine the flour, sugars, salt, oatmeal, and butter in the bowl of an electric mixer fitted with the paddle attachment. Mix on low speed for 1 min, until the mixture is in large crumbles. Sprinkle evenly over the fruit, covering the fruit completely.
3) Place the baking dish on a sheet pan and bake for 50 mins to 1 hour, until the top is brown and the fruit is bubbly. Serve warm.