Preheat your oven to 350°F and lightly grease an 8x8 inch or 9x13 inch baking dish with butter or cooking spray. While the oven heats, measure out all your ingredients and have them ready—this ensures smooth cooking with no scrambling.

In a medium bowl, combine the raspberries, 1/2 cup brown sugar, cornstarch, fresh lemon juice, vanilla extract, and lemon zest. Gently toss everything together until the raspberries are evenly coated and the cornstarch is fully incorporated—this thickens the fruit juices as it bakes. Transfer the mixture to your prepared baking dish, spreading it into an even layer.

In a separate bowl, whisk together the oats, all-purpose flour, 1/2 cup packed brown sugar, pumpkin pie spice, and fine sea salt. Add the chilled, cubed butter and use a fork or your fingertips to work it into the dry mixture until it resembles coarse breadcrumbs with pea-sized pieces of butter throughout. I prefer using my fingertips for better control—it gives you a more rustic, chunky texture that bakes up beautifully crispy.

Spread the crumble topping from Step 3 evenly over the raspberry filling, breaking up any large clumps as you go. Don't pack it down—keeping it loose allows it to bake up light and crunchy. Place in the preheated 350°F oven and bake for 30–35 minutes, until the topping is golden brown and the fruit begins to bubble slightly around the edges.

Remove from the oven and let the crumble cool for 10–15 minutes on a wire rack—this brief resting time allows the filling to set slightly so it won't run all over your plate. Serve warm, ideally with a dollop of whipped cream or vanilla ice cream to contrast the tart raspberries and sweet, buttery crumble.