Melt the unsalted butter in a light-colored saucepan over medium heat. Swirl the pan occasionally as the butter foams and crackles. After 4 to 5 minutes, golden-brown specks will form at the bottom with a deeply nutty, toasty aroma. Immediately pour into a heatproof bowl to stop the cooking and let cool for 5 minutes.

Preheat your oven to 375°F (190°C). Grease a 10-inch cast iron skillet or 9x9-inch ceramic baking dish with butter.

In a large mixing bowl, toss the sliced Honeycrisp and Granny Smith apples with lemon juice, maple syrup, brown sugar, cinnamon, nutmeg, and flour until evenly coated. Spread the apples evenly into the prepared skillet.

In a separate bowl, whisk together rolled oats, flour, dark brown sugar, chopped pecans, cinnamon, and sea salt. Pour in the warm browned butter and vanilla extract. Stir with a fork until moist, clumpy streusel clusters form.

Scatter the brown butter oat streusel clusters generously and evenly over the spiced apples, covering the fruit completely.

Bake at 375°F (190°C) for 32 to 35 minutes until the oat streusel is deeply golden and crisp, and the spiced apple juices are bubbling vigorously around the edges.

Remove from oven and let rest for 10 minutes so the caramel juices thicken slightly. Serve warm in shallow bowls with a generous scoop of vanilla bean ice cream and an optional drizzle of warm salted caramel.