Preheat your oven to 350°F (175°C) to ensure it's ready for baking.
In a large mixing bowl, gently combine the quartered strawberries, blueberries, and raspberries.
Add the granulated sugar, fresh lemon juice, and pure vanilla extract to the berries. Stir gently to ensure that each berry is well coated without mashing them.
In a separate bowl, whisk together the rolled oats, all-purpose flour, packed brown sugar, ground cinnamon, and kosher salt until evenly mixed.
Gradually pour the melted butter into the dry mixture, stirring as you go, until all the ingredients are coated and form a crumbly texture.
Grease a 9x9-inch baking dish, then spread the berry mixture evenly across the bottom.
Evenly sprinkle the oat topping over the berry layer, making sure to cover all the fruit completely.
Place the dish in the preheated oven and bake for 30-35 minutes, or until the topping turns a golden brown and the berries begin to bubble around the edges.
Once baked, remove from the oven and let it cool for at least 10 minutes before serving to allow the juices to set.
Notes
Serve warm with vanilla ice cream and a drizzle of honey for added sweetness.