Preheat & Prep Baking Dish: Preheat your oven to 350°F (175°C). Generously grease an 8-inch square ceramic baking dish or a 9-inch deep pie dish with butter.

Toss the Apple Filling: In a large mixing bowl, toss the sliced apples with lemon juice, granulated sugar, 2 tablespoons brown sugar, ground cinnamon, nutmeg, flour, and vanilla extract until every slice is evenly coated in spices. Spread the apples evenly across the prepared baking dish.

Prepare the Oat Streusel: In a separate medium bowl, whisk together the rolled oats, flour, 1/2 cup brown sugar, cinnamon, and sea salt. Add the cold cubed butter. Use a pastry cutter, fork, or clean fingertips to work the butter into the dry ingredients until coarse, pea-sized buttery crumbs and clumps form.

Assemble & Scatter Topping: Scatter the oat streusel mixture evenly over the apples, pressing down very lightly so the topping clings to the apples while keeping plenty of loose, buttery crumble clusters.

Bake to Deep Golden Perfection: Bake uncovered at 350°F (175°C) for 40 to 45 minutes, until the oat topping is crisp, fragrant, and deep golden brown, and the spiced apple juices are bubbling vigorously along the edges.

Cool Slightly & Serve: Let the apple crisp cool on a wire rack for 10 minutes so the caramelized juices settle slightly. Spoon generous warm servings into bowls and top immediately with a scoop of melting vanilla bean ice cream and a warm caramel drizzle!