Preheat oven & prep dish: Preheat your oven to 350°F (175°C). Lightly grease an 8x8-inch or 9x9-inch ceramic baking dish (or 2-quart baking dish) with butter or cooking spray.
Toss spiced apple filling: In a large mixing bowl, toss the sliced apples with lemon juice, granulated sugar, brown sugar, cinnamon, nutmeg, flour, and vanilla extract until every slice is evenly coated in spices. Transfer into the prepared baking dish and spread into an even layer.
Make buttery oat streusel: In the same bowl (no need to wash), whisk together rolled oats, flour, brown sugar, cinnamon, and salt. Add the cold cubed butter. Using a pastry cutter, fork, or clean fingertips, cut the butter into the dry mixture until coarse, pea-sized buttery crumbs and clumps form.
Top generously: Scatter the oat streusel crumble evenly over the entire surface of the apples, pressing down gently so it adheres. (Do not pack tightly; loose clumpy streusel bakes up extra crunchy!).
Bake until golden & bubbly: Bake at 350°F (175°C) for 35 to 40 minutes, until the oat crumble is deep golden-brown and fragrant, and spiced apple juices are visibly bubbling up around the edges of the dish. (If the topping browns too fast, loosely tent with foil for the last 10 minutes).
Cool slightly & serve warm: Remove from the oven and let rest on a wire rack for 10 minutes to allow the caramel apple juices to set. Scoop generous warm portions into dessert bowls, crown with a large scoop of vanilla bean ice cream, and drizzle with warm salted caramel sauce. Serve warm!
