Preheat oven to 350°F (175°C). Lightly butter a 9x9-inch ceramic baking dish or 10-inch cast iron skillet.

In a large mixing bowl, toss the sliced apples with lemon juice, brown sugar, flour, ground cinnamon, nutmeg, and vanilla extract until every slice is evenly glazed. Transfer apples into the prepared baking dish and spread into an even, compact layer.

In a separate bowl, whisk together rolled oats, flour, dark brown sugar, cinnamon, and fine sea salt.

Add the cold diced butter cubes. Using your fingertips or a pastry cutter, rub the butter into the dry oat mixture until large, crumbly, pea-sized clusters form. Avoid overworking into a paste; distinct butter pebbles ensure maximum crispness.

Scatter the oat streusel evenly over the spiced apples, covering the fruit completely right to the edges.

Bake uncovered at 350°F (175°C) for 35 to 40 minutes, until the oat topping is deep golden brown and crunchy, and the apple juices are bubbling vigorously along the edges of the dish.

Remove from the oven and let cool for 10 minutes to allow the bubbling spiced fruit juices to thicken into a silky glaze.

Drizzle warm salted caramel sauce generously over the golden crust, then sprinkle with flaky sea salt. Scoop into warm dessert bowls and crown with a generous scoop of cold vanilla bean ice cream. Serve immediately!