6tablespoonscold unsalted buttercut into small cubes
Instructions
Prep & Preheat
Preheat your oven to 350°F (175°C). Lightly butter a 9x13-inch baking dish or an equivalent 2.5–3 quart baking dish and set aside.
Make the Berry Filling
In a large bowl, combine the strawberries, blueberries, and raspberries. Add the granulated sugar, cornstarch, lemon juice, lemon zest, and vanilla extract. Toss gently until all the berries are evenly coated.
Pour the berry mixture into the prepared baking dish and spread into an even layer.
Make the Oat Crumble Topping
In a medium bowl, whisk together the rolled oats, flour, brown sugar, cinnamon, and salt until combined.
Add the cold butter cubes. Using your fingertips, work the butter into the dry ingredients by pressing and rubbing until the mixture resembles coarse crumbs with some pea-sized clumps of butter remaining. Do not overwork — those butter chunks create the crispy texture.
Assemble & Bake
Scatter the oat crumble topping evenly over the berry filling, covering the berries completely.
Bake for 35–40 minutes (add 5 minutes if using frozen berries) until the crumble topping is deep golden brown and the berry filling is visibly bubbling around the edges and through the topping.
Remove from the oven and let rest for 10–15 minutes before serving. This allows the filling to thicken slightly. Serve warm with vanilla ice cream or freshly whipped cream.
Notes
Make-Ahead: The oat crumble topping can be made up to 5 days ahead and stored in an airtight container in the refrigerator. Storage: Cover and refrigerate leftovers for up to 4 days. Reheat in a 350°F oven for 10–12 minutes to restore crunch. Freezing: Assemble unbaked, cover tightly, and freeze for up to 3 months. Bake from frozen at 350°F for 55–65 minutes. Gluten-Free: Use certified GF oats and a 1:1 GF flour blend in the topping.