Step 1: Prepare the fruit filling. In a large bowl, combine the sliced peaches, mixed berries, 1/4 cup granulated sugar, cornstarch, vanilla extract, and 1/2 tsp ground cinnamon. Toss gently to coat the fruit evenly. Set aside.
Step 2: Prepare the oat crumble topping. In a separate medium bowl, combine the rolled oats, all-purpose flour, brown sugar, 1/4 cup granulated sugar, 1/2 tsp ground cinnamon, and salt.
Step 3: Add the cold, cubed butter to the dry ingredients for the topping. Using your fingertips or a pastry blender, cut the butter into the mixture until it resembles coarse crumbs with some pea-sized pieces remaining. Avoid overmixing to keep the topping tender.
Step 4: Preheat your Blackstone griddle to medium-low heat. You want enough heat to caramelize the fruit without burning.
Step 5: Melt 1 tablespoon of butter directly on the griddle surface. Once melted, spread it around a designated cooking area.
Step 6: Spoon the fruit filling onto the buttered section of the griddle. Spread it out into a single, even layer. Cook for about 8-10 minutes, stirring occasionally, until the peaches soften slightly and the berries begin to burst, and the juices start to thicken.
Step 7: Melt the remaining 1 tablespoon of butter in another section of the griddle, or push the fruit aside and add it there. Sprinkle the oat crumble topping onto the melted butter. Spread it out thinly. Cook for about 5-7 minutes, stirring occasionally, until the crumble is golden brown and fragrant. Watch carefully, as it can burn quickly.
Step 8: Once the fruit is tender and bubbly, and the crumble is golden brown and crispy, gently combine the cooked fruit with the crispy crumble directly on the griddle using two spatulas. Mix just enough to incorporate the crumble throughout the fruit.
Step 9: Remove the Peach and Berry Crisp from the griddle immediately and serve warm. It's best served fresh off the griddle. For any leftovers, consider storing them in Basics Glass Food Storage containers once cooled, then reheat gently. Enjoy this delightful addition to your collection of blackstone dessert recipes!