Preheat the oven to 350°F. Lightly grease an 9x9-inch (or similar) baking dish.
In a medium bowl, combine the oats, butter, flour, sugars, salt, and cinnamon. Use a pastry cutter or your hands to blend until the mixture forms coarse crumbs with no dry flour spots. Refrigerate while you make the fruit filling.
1 cup old-fashioned rolled oats, 6 tablespoons unsalted butter, ½ cup all-purpose flour, ½ cup light brown sugar, ¼ cup granulated sugar, ½ teaspoon salt, ½ teaspoon ground cinnamon
In a separate bowl, toss the sliced pears with lemon juice, both sugars, cornstarch, cinnamon, and nutmeg until evenly coated. Pour the mixture into the prepared dish and spread into an even layer.
6 medium pears, 2 tablespoons freshly squeezed lemon juice, ¼ cup granulated sugar, ¼ cup light brown sugar, 2 tablespoons cornstarch, ½ teaspoon ground cinnamon, ¼ teaspoon ground nutmeg
Sprinkle the topping evenly over the pears.
Bake for 35–45 minutes, until the topping is golden brown and the filling is bubbling thickly around the edges.
Cool for 10–15 minutes before serving.