Prep your oven and pan. Heat the oven to 350°F (175°C). Lightly butter a 9-inch square baking dish or a similar 2-quart dish.
Slice the apples evenly. Peel, core, and slice the apples about 1/4-inch thick. Even slices cook evenly and help prevent a mushy filling.
Toss the filling. In a large bowl, combine the apple slices with granulated sugar, brown sugar, lemon juice, vanilla, cinnamon, nutmeg, cloves (if using), cornstarch, and a pinch of salt. Stir until the apples are coated and glossy.
Make the crumble. In a separate bowl, mix flour, oats, 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 pea-sized clumps. Stir in nuts if using. Don’t overwork; you want a sandy, clumpy texture.
Assemble. Spread the apples evenly in the prepared dish, scraping in any juices. Sprinkle the crumble evenly over the top, leaving some larger clumps for extra crunch.
Bake. Place the dish on a rimmed baking sheet (to catch any drips) and bake for 40–50 minutes. It’s done when the topping is golden brown and the filling is bubbling around the edges.
Rest before serving. Let the crumble cool for at least 15–20 minutes. This helps the juices thicken so each scoop holds together.
Serve. Spoon into bowls and top with vanilla ice cream or whipped cream, if you like.