Preheat your oven to 350°F (175°C). Lightly grease a deep 9x9-inch square ceramic baking dish or 2.5-quart oval baking dish with a tablespoon of softened butter.

Peel, core, and slice the apples into uniform 1/4-inch thick slices. In a large mixing bowl, toss the sliced apples with granulated sugar, lemon juice, ground cinnamon, nutmeg, flour, and vanilla extract until every slice is evenly coated in spices. Transfer the spiced apples into the prepared baking dish and spread them into an even layer.

In a separate medium bowl, combine the rolled oats, flour, packed dark brown sugar, cinnamon, and sea salt. Stir well to break up any brown sugar lumps.

Add the cold butter cubes to the oat mixture. Using a pastry blender or your clean fingertips, work the cold butter into the dry ingredients until coarse, pea-sized clumps and crumbly clusters form. (Do not overmix into a smooth paste; you want visible buttery oat clusters for maximum crunch).

Evenly scatter the oat streusel topping over the spiced apples, covering the fruit completely from edge to edge.

Bake at 350°F (175°C) for 40 to 45 minutes, until the oat topping is golden brown and crisp, and the amber apple juices are bubbling vigorously along the edges of the dish. (If the streusel browns too quickly around minute 30, loosely tent foil over top).

Remove from the oven and let rest on a wire rack for 10 minutes so the bubbling caramel apple juices thicken slightly. Spoon generous warm helpings into dessert bowls, crown immediately with a generous scoop of vanilla bean ice cream, drizzle with warm salted caramel, and enjoy!