Prep the oven and pan: Heat the oven to 350°F (175°C). Grease a 9-inch square baking dish or a 2-quart casserole with butter or nonstick spray.

Prep the apples: Peel, core, and slice the apples about 1/4-inch thick. Toss with lemon juice, granulated sugar, 1 teaspoon cinnamon, a pinch of salt, vanilla, and cornstarch. Spread evenly in the baking dish.

Mix the dry topping: In a bowl, combine flour, oats, brown sugar, remaining 1/2 teaspoon cinnamon, nutmeg, and 1/2 teaspoon salt. Stir until no clumps of sugar remain.

Add butter and starter: Pour in the melted butter and sourdough starter. Stir until the mixture turns into moist clumps. It should hold together when squeezed but still crumble apart.

Add nuts or fruit (optional): Fold in chopped nuts or dried cranberries for extra crunch or tartness.

Top the apples: Scatter the crumb mixture evenly over the apples, leaving some larger clusters for extra texture.

Bake: Place the dish on a sheet pan to catch any bubbling juices. Bake for 40–50 minutes, until the topping is deep golden brown and the apple juices are bubbling around the edges.

Rest before serving: Let the crisp sit 10–15 minutes. This helps the juices thicken so each scoop holds together.

Serve: Enjoy warm with ice cream, whipped cream, or a drizzle of caramel sauce.