Preheat the oven to 350°F (175°C). Grease an 8x8 inch baking dish with cooking spray or butter.
In a large mixing bowl, combine the sliced strawberries, blueberries, and raspberries.
Add sugar, lemon juice, and vanilla extract to the berries. Toss gently until the berries are well coated. Pour this mixture into the prepared baking dish.
In another bowl, combine the rolled oats, all-purpose flour, brown sugar, cinnamon, and a pinch of salt. Mix to combine.
Drizzle melted butter over the mixture and mix until crumbly.
Evenly spread the oat mixture over the berry layer in the baking dish.
Bake in the preheated oven for 25-30 minutes, or until the top is golden brown and the berries are bubbling.
Remove from oven and let cool for about 10 minutes before serving.
Notes
Serve warm, topped with a scoop of vanilla ice cream or whipped cream for an indulgent treat.