Preheat the oven to 350°F (175°C) if you plan to bake the crumble immediately.

In a large mixing bowl, whisk together the flour, rolled oats, brown sugar, granulated sugar, cinnamon, and salt until evenly combined.

Add the cold, cubed butter to the dry mixture and toss lightly to coat the butter with flour.

Using your fingertips, a fork, or a pastry cutter, rub or cut the butter into the dry ingredients until the mixture resembles coarse breadcrumbs with pea-sized bits of butter remaining.

Drizzle in the vanilla extract (if using) and gently mix to incorporate.

Fold in the chopped nuts if desired, distributing them evenly throughout the topping.

Squeeze a handful of the mixture; it should clump lightly and crumble when tapped. If it does, the texture is correct.

Scatter the crumble evenly over your prepared fruit filling in a 9-inch baking dish, taking care not to press it down.

Bake at 350°F (175°C) for 25 minutes, or until the topping is golden brown and the fruit juices bubble at the edges. Serve warm.