Begin by melting the unsalted butter in a medium saucepan over medium heat. Stir frequently until the butter foams, turns golden brown with brown flecks, and smells nutty. Remove from heat immediately and set aside to cool slightly.
While the butter cools, peel and slice your ripe pears. Place them in a mixing bowl and gently toss with the vanilla extract, cinnamon, nutmeg, and a pinch of salt to infuse flavor.
In another bowl, combine the all-purpose flour, rolled oats, and brown sugar. Pour in the slightly cooled browned butter and mix with a wooden spoon or your fingers until the mixture resembles coarse crumbs.
Preheat your oven to 350°F (175°C). Spread the spiced pears evenly in the bottom of your baking dish. Sprinkle the crumble topping generously over the pears, ensuring the fruit is fully covered.
Bake for 35-40 minutes, or until the topping is golden brown and crisp, and the pears are bubbly and soft underneath.
Allow the crumble to cool slightly before serving. Enjoy on its own or with vanilla ice cream or whipped cream.