Melt 1 stick of unsalted butter in a light-colored saucepan over medium heat. Swirl the pan occasionally as the butter melts, foams, and crackles. After 4 to 5 minutes, golden-brown flecks will settle on the bottom and a heavenly nutty hazelnut aroma will arise. Remove from the heat immediately and pour into a heat-safe bowl so it stops cooking.

Preheat your oven to 350°F (175°C) and grease a 9x9-inch ceramic baking dish or a 10-inch cast iron skillet with butter. In a large bowl, toss the sliced Honeycrisp and Granny Smith apples with brown sugar, fresh lemon juice, flour, ground cinnamon, nutmeg, vanilla extract, and sea salt until every apple slice is glossily coated. Spread into the prepared baking dish.

In a separate medium bowl, whisk together the rolled oats, flour, brown sugar, chopped pecans, ground cinnamon, and flaky sea salt. Pour in the warm brown butter with all the toasted milk solids. Stir with a fork or wooden spoon until clumpy streusel clusters form.

Scatter the brown butter oat crumble evenly across the spiced apples, leaving no bare fruit showing. Bake uncovered at 350°F for 40 to 45 minutes until the streusel is deeply golden and crisp, and the spiced apple juices are bubbling vigorously up the sides of the dish. Rest for 10 minutes before serving warm with scoops of vanilla bean ice cream and salted caramel drizzle.