In a medium bowl, combine the strawberries, blueberries, raspberries, lemon juice, and honey or maple syrup. Gently toss to coat the berries and set aside.
In another bowl, mix together the rolled oats, flour, brown sugar, cinnamon, and salt.
Add the cubed cold butter to the oat mixture. Using your fingers or a pastry cutter, blend until the mixture resembles coarse crumbs.
In a greased 8x8-inch baking dish, spread the berry mixture evenly across the bottom.
Evenly sprinkle the oat topping over the berries, covering them completely.
Bake in the preheated oven for 30-35 minutes, or until the topping is golden brown and the berries are bubbling.
Remove from the oven and let it cool slightly.
Notes
Serve warm with a scoop of vanilla ice cream on top, garnished with a sprig of mint.