Melt 1/2 cup (1 stick) unsalted butter in a light-colored stainless steel skillet over medium heat. Swirl the pan occasionally as the butter melts, foams, and begins to crackle. After 4 to 5 minutes, golden brown specks will form on the bottom and a rich, toasted nutty aroma will emerge. Immediately pour the browned butter into a heatproof bowl, scraping in all the toasted milk solids. Stir in 1 teaspoon pure vanilla extract and let cool for 5 minutes.

Preheat your oven to 350°F (175°C) and lightly butter a 2-quart oval ceramic baking dish or 9x9-inch baking dish. In a large mixing bowl, toss the sliced Honeycrisp and Granny Smith apples with maple syrup, brown sugar, lemon juice, cinnamon, nutmeg, and 1 tablespoon flour until every slice is glossy and spiced. Transfer apples into the prepared dish and spread evenly.

In a medium bowl, whisk together the rolled oats, flour, brown sugar, chopped pecans, cinnamon, and fine sea salt. Pour the warm browned butter over the dry mixture. Use a fork or clean fingertips to toss until clusters and clumps form. Scatter the streusel evenly across the apples, ensuring plenty of generous clusters remain intact.

Bake on the center rack at 350°F (175°C) for 38 to 42 minutes, until the oat pecan streusel is deeply golden and crisp, and the spiced apple juices are vigorously bubbling around the edges. (If the pecans brown too quickly at minute 30, tent loosely with foil).

Remove from the oven and let stand for 10 minutes so the caramel juices thicken slightly. Spoon generous warm portions into shallow bowls, crown immediately with a cold scoop of vanilla bean ice cream, drizzle with warm salted caramel, and serve!