Preheat the oven to 375°F (190°C).
In a large bowl, combine the sliced apples, sliced pears, granulated sugar, all-purpose flour, cinnamon, nutmeg, and salt. Toss until the fruit is coated with the sugar and spices.
Transfer the fruit mixture to a 9x13-inch baking dish.
In a separate bowl, combine the oats, brown sugar, and diced cold butter. Use your fingers or a pastry cutter to mix the ingredients until the mixture resembles coarse crumbs.
Spread the oat topping evenly over the fruit mixture in the baking dish.
Bake for 35-40 minutes, or until the fruit is tender and the topping is golden brown and crispy.
Remove from the oven and let cool for a few minutes before serving.
Serve warm with vanilla ice cream or whipped cream, if desired.