01 - Preheat the oven to 350°F (175°C).
02 - In a large bowl, combine the sliced pears with grated ginger, ground cinnamon, ground nutmeg, light brown sugar, lemon juice, cornstarch, and a pinch of salt. Toss gently until all pear slices are evenly coated.
03 - Transfer the pear mixture into a greased 9-inch baking dish, spreading it into an even layer.
04 - In a separate mixing bowl, combine the all-purpose flour, rolled oats, light brown sugar, granulated sugar, ground cinnamon, ground ginger, and salt. Add the cold, cubed unsalted butter. Using your fingertips or a pastry blender, work the butter into the dry ingredients until the mixture resembles coarse crumbs. If using, gently stir in the chopped nuts.
05 - Evenly distribute the crumble topping over the pear mixture in the baking dish.
06 - Place the dish in the preheated oven and bake for 35 minutes, or until the topping achieves a golden-brown hue and the fruit filling is visibly bubbling.
07 - Allow the crumble to cool for 10 minutes before serving. Present warm, with optional accompaniments such as vanilla ice cream or whipped cream.