Step 1: In a large bowl, gently combine sliced peaches, mixed berries, granulated sugar, cornstarch, vanilla extract, and ground cinnamon. Toss until the fruit is evenly coated. Set aside.
Step 2: In a separate medium bowl, combine old-fashioned rolled oats, all-purpose flour, packed light brown sugar, salt, and ground nutmeg. Whisk these dry ingredients together until well mixed.
Step 3: Add the cold unsalted butter, cut into small cubes, to the dry ingredients. Work the butter into the mixture using your fingertips, a pastry blender, or a fork until it resembles coarse crumbs with some pea-sized pieces remaining. Place this mixture in the refrigerator.
Step 4: Preheat your Blackstone griddle to medium-low heat (approximately 250-300°F or 120-150°C). Add 1 tablespoon of butter to the griddle surface and let it melt, spreading it thinly.
Step 5: Carefully spoon the fruit mixture onto the buttered section of the griddle. Spread it into an even layer, approximately 1-inch thick, forming your desired shape.
Step 6: Evenly sprinkle the chilled crumble topping over the fruit layer. Cover the crumble with a high-domed griddle cover or a large inverted foil pan.
Step 7: Cook for 15-20 minutes, or until the fruit is tender and bubbling, and the topping is golden brown and crisp. Check periodically, adjusting heat as needed to prevent burning.
Step 8: Once cooked, carefully remove the crumble from the griddle to a serving platter or scoop individual portions. Let it cool for a few minutes before serving warm with ice cream or whipped cream.