Preheat Oven & Prep Dish: Get your oven ready by preheating it to 350°F (175°C). Lightly grease a 9-inch baking dish and set it aside.
Mix the Berry Filling: In a medium bowl, gently combine your chosen berries, cornstarch, and lemon juice. Give it a good stir, then let it sit for about 15 minutes, stirring occasionally, to let the flavors marry.
Layer the Berries: Pour the berry mixture evenly into the bottom of your prepared baking dish.
Create the Crumble Topping: In a separate medium bowl, whisk together the flour, both sugars (brown and granulated), cinnamon, and salt until everything is well combined.
Cut in the Butter: Use a pastry cutter or two forks to 'cut in' the cold butter until the mixture forms coarse, crumbly bits. If you're adding nuts, stir them in now.
Top it Off: Generously sprinkle the crumble mixture evenly over the berry filling in your dish.
Bake Until Golden: Bake for 20 to 25 minutes, or until the berry filling is bubbling deliciously and the topping is golden brown and crisp.
Cool & Serve: Let the crumble cool for about 15 minutes before serving. This short cooling period helps the fruity filling set perfectly. Enjoy it warm, perhaps with a dollop of Greek yogurt or a light scoop of vanilla ice cream!