1. Preheat your oven to 375 F (190 C). Lightly grease an 8×8 inch baking dish or a similar-sized oven-safe dish with butter or cooking spray.

2. In a large bowl, gently combine the 4 cups of mixed berries with 1/4 cup granulated sugar, 1 tablespoon cornstarch, and 1 teaspoon lemon juice. Toss everything together until the berries are evenly coated.

3. Pour the berry mixture into your prepared baking dish, spreading it out into an even layer.

4. In a separate medium bowl, combine the 1 cup rolled oats, 1/2 cup all-purpose flour, 1/4 cup light brown sugar, 1/4 teaspoon ground cinnamon, and 1/4 teaspoon salt. Whisk these dry ingredients together until well combined.

5. Add the 1/4 cup cold unsalted butter, cut into small pieces, to the dry topping mixture. Using your fingertips, a pastry blender, or two knives, cut the butter into the mixture until it resembles coarse crumbs with some pea-sized pieces of butter remaining. If the mixture seems too dry to form crumbs, you can add 1 tablespoon of cold water, a teaspoon at a time, and mix until it just comes together.

6. Evenly sprinkle the oat topping over the berry mixture in the baking dish. Make sure the berries are mostly covered, but don’t press the topping down too firmly. A loose, crumbly topping will bake up crispier.

7. Place the baking dish on a baking sheet (to catch any potential drips) and bake for 25-30 minutes. You’ll know it’s done when the topping is golden brown and the berry filling is bubbly around the edges.

8. Remove the crisp from the oven and let it cool for at least 10-15 minutes before serving. This allows the berry juices to set slightly, preventing a soupy consistency.