Heat the oven to 350°F (175°C).
Lightly butter a 9x13-inch baking dish or similar 3-quart casserole.
Prep the apples. Peel (optional), core, and slice into 1/4-inch thick slices. Aim for even pieces so they cook at the same rate.
Toss the filling. In a large bowl, combine apples with 1/3 cup granulated sugar, 1 tablespoon brown sugar, 1 tablespoon lemon juice, 1 teaspoon lemon zest, 1–1 1/2 teaspoons cinnamon, a pinch of nutmeg, 1 teaspoon vanilla, 1/4 teaspoon salt, and 1 1/2 tablespoons cornstarch. Stir until the slices are glossy and evenly coated.
Make the crumble. In another bowl, mix 1 cup rolled oats, 3/4 cup all-purpose flour, 1/2 cup packed brown sugar, 1/4 teaspoon salt, and 1/2 teaspoon cinnamon.
Cut in 1/2 cup (1 stick) cold unsalted butter, cubed. Use your fingers or a pastry cutter until you have pea-sized crumbs. If using nuts, fold in 1/2 cup now.
Assemble. Spread the apples evenly in the baking dish.
Sprinkle the crumble topping over the fruit, leaving some gaps so steam can escape and the topping can crisp.
Bake. Place on the middle rack and bake 40–50 minutes, until the topping is deep golden and the filling is bubbling around the edges. If the top is browning too quickly, tent loosely with foil toward the end.
Rest and serve. Let it cool for at least 15 minutes so the juices thicken. Serve warm with ice cream, whipped cream, or a drizzle of cold heavy cream.