Preheat the oven to 350°F. Lightly grease a 12-inch oven-safe skillet or a 9x13-inch baking dish.
Peel, core, and slice the apples into even ¼- to ⅓-inch slices. Place them in a large mixing bowl.
Add the lemon juice, vanilla extract, cinnamon, nutmeg, arrowroot starch or cornstarch, and brown sugar. Toss until the apples are evenly coated.
Transfer the apple mixture to the prepared skillet or baking dish and spread into an even layer.
In a separate bowl, combine the melted butter, rolled oats, flour, chopped pecans, brown sugar, cinnamon, and sea salt. Stir until the mixture is evenly moistened and crumbly.
Sprinkle the topping evenly over the apples without pressing it down too firmly.
Bake uncovered for 45–60 minutes, or until the topping is golden brown, the filling is bubbling around the edges, and the apples are fork-tender.
Let rest for 10–15 minutes before serving. Enjoy warm!