Preheat your oven to 350°F (175°C). Lightly butter a 9x9-inch or 2-quart ceramic baking dish.

In a small light-colored skillet, melt unsalted butter over medium heat. Swirl constantly for 4 to 5 minutes as it foams and crackles. Once golden amber with fragrant nutty brown flecks at the bottom, immediately remove from heat and let cool for 5 minutes.

In a large bowl, toss apple slices with lemon juice, maple syrup, cornstarch, ground cinnamon, nutmeg, and vanilla extract until every slice is glossily coated. Transfer to the prepared baking dish and spread into an even layer.

In a separate bowl, whisk together rolled oats, flour, brown sugar, cinnamon, sea salt, and chopped pecans.

Pour the cooled brown butter over the dry oat mixture. Stir with a fork until coarse, clumpable crumbs form.

Scatter the streusel generously and evenly over the spiced apples, gently pressing down so the crumbs adhere.

Bake on the center oven rack for 40 to 45 minutes until the spiced apple juices bubble thick around the edges and the oat streusel is deep golden-brown and crisp.

Cool for 10 to 15 minutes before spooning into dessert bowls. Serve warm topped with a large scoop of vanilla bean ice cream or freshly whipped cream.