Preheat your oven to 350°F (175°C) and grease a baking dish (8x8 inches works great).
In a large mixing bowl, combine the sliced pears with cinnamon, nutmeg, and lemon juice. Mix well and set aside to allow the flavors to meld.
In another bowl, combine the rolled oats, flour, brown sugar, granulated sugar, and salt. Stir in the melted butter until the mixture is crumbly and well incorporated. If using, add the chopped walnuts for extra texture.
Spread the spiced pear mixture evenly in the greased baking dish.
Evenly distribute the crumble topping over the pears, making sure to cover them completely.
Bake in the preheated oven for about 30-35 minutes, or until the topping is golden brown and the pears are bubbling.
Remove the crumble from the oven and allow it to cool slightly before serving.
Notes
Serve warm with a scoop of vanilla ice cream on top for a delightful contrast. Dust with a sprinkle of cinnamon for added flair.