Step 1: Prepare the Apples: In a large bowl, toss the sliced apples with lemon juice, 1/2 cup brown sugar, 1 teaspoon cinnamon, nutmeg, and a pinch of salt. Set aside.
Step 2: Make the Crumble Topping: In a separate medium bowl, combine flour, rolled oats, granulated sugar, 1/4 cup brown sugar, 1/2 teaspoon cinnamon, and 1/4 teaspoon salt. Cut in the cold butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.
Step 3: Preheat Griddle: Preheat your Blackstone griddle to medium-low heat (approximately 300-325°F).
Step 4: Cook the Apples: Add 2 tablespoons of butter to the preheated griddle and let it melt. Pour the apple mixture onto the griddle, spreading it into an even layer. Cook, stirring occasionally, for 10-15 minutes, or until apples are tender-crisp and the sauce has thickened and caramelized.
Step 5: Add Crumble Topping: Evenly sprinkle the prepared crumble topping over the cooked apples. Do not press down.
Step 6: Bake on Griddle: Reduce griddle heat to low. Cover the apple crumble with a dome lid or a large metal bowl to create an oven effect. Cook for another 10-15 minutes, or until the crumble topping is golden brown and crisp. Check periodically to ensure it's not burning.
Step 7: Rest and Serve: Carefully remove the dome. Let the griddled apple crumble rest for 5-10 minutes before serving. Serve warm, perhaps with a scoop of vanilla ice cream or whipped cream.