Position a rack in the middle and heat the oven to 350°F. Butter a 9x13-inch baking dish (a 2-quart dish). A light butter coat helps the filling’s sugary edges lift out cleanly later.

Peel, core, and slice the apples 1/4-inch thick — thick enough to stay tender-firm through baking, thin enough to cook evenly. In the largest bowl you own, toss the slices with the granulated sugar, flour, cinnamon, nutmeg, lemon juice, vanilla, and salt until every slice is evenly coated. Spread the filling in an even layer in the prepared dish.

In a medium bowl, whisk the oats, flour, brown sugar, cinnamon, and salt. Scatter the cold butter cubes over the top and cut them in with a pastry cutter or your fingertips until the mixture forms chunky, pea-size clumps with some sandy bits — the clumps are what bake into crisp nuggets. Stir in the pecans, if using.

Crumble the topping evenly over the apples, covering edge to edge. Don’t press it down — loose clumps bake up crispier.

Bake 40–45 minutes, until the topping is deep golden brown and the juices are thick and bubbling — check the center, not just the edges. If the topping is browning faster than the filling is bubbling, tent loosely with foil for the last 10–15 minutes.

Cool at least 10 minutes before serving — this resting time is in addition to the 65-minute total. The filling thickens as it stands, so the first scoop holds together instead of running. Serve warm, with vanilla ice cream.