Preheat the oven: Heat to 350°F (175°C).
Lightly butter a 9x9-inch baking dish or a similar 2–2.5 quart dish.
Prep the apples: Peel, core, and slice the apples into even 1/4-inch slices. Uniform slices help them bake evenly.
Mix the filling: In a large bowl, toss apples with granulated sugar, brown sugar, cinnamon, nutmeg, lemon juice, vanilla, flour or cornstarch, and a pinch of salt until evenly coated.
Make the topping: In another bowl, combine oats, flour, brown sugar, granulated sugar, cinnamon, and salt. Add the cold butter cubes.
Use your fingers or a pastry cutter to work the butter in until the mixture forms moist clumps with pea-sized bits. Stir in nuts if using.
Assemble: Spread the apple mixture in the prepared dish. Sprinkle the topping evenly over the apples, breaking up any large clumps.
Bake: Bake for 40–50 minutes, until the topping is deeply golden and the filling is bubbling at the edges.
If the top browns too quickly, tent loosely with foil for the last 10 minutes.
Rest: Let the crisp cool for at least 15 minutes. This helps the juices thicken so the servings hold together.
Serve: Spoon warm crisp into bowls and top with ice cream or whipped cream if you like.