Preheat oven to 350°F (175°C). Grease a 9-inch cast iron skillet or deep 8x8-inch ceramic baking dish with butter.

Peel, core, and slice the apples evenly into 1/4-inch thick wedges. In a large bowl, toss apple slices with fresh lemon juice, pure maple syrup, granulated sugar, all-purpose flour, ground cinnamon, nutmeg, and vanilla extract until every slice is glossy and evenly coated in warm spices.

Transfer the spiced apple mixture into the prepared skillet or baking dish, arranging them in a slightly compact, even layer.

In a separate medium bowl, prepare the crunchy crumble topping: whisk together old-fashioned rolled oats, flour, dark brown sugar, cinnamon, and sea salt. Add the cold cubed butter. Use your fingertips or a pastry cutter to work the butter into the dry mixture until moist, pea-sized clumpy clusters form.

Evenly scatter the clumpy oat streusel across the entire apple layer from edge to edge, leaving some coarse pebbles intact for maximum baked crunch.

Bake at 350°F (175°C) for 42 to 48 minutes until the spiced caramel juices are vigorously bubbling around the edges and the oat crumble topping is deeply golden brown and crisp.

Remove from the oven and let rest for 10 minutes to allow the bubbling caramel juices to thicken slightly. Spoon warm into bowls, top generously with a scoop of vanilla bean ice cream, and watch it melt into the warm spiced caramel apples!