Preheat oven to 350°F. Lightly grease a 9 x 13-inch ceramic or glass baking dish. Avoid using a metal pan—it can give the fruit a metallic taste.
Peel and cut apples into 1/2-inch cubes. In a large pot, toss cubed apples with cornstarch. Add remaining filling ingredients and cook over medium heat for 5 minutes, stirring consistently, until apples soften slightly but hold shape. Transfer apples to the greased dish and bake for 10 minutes.
Combine room-temperature melted butter with crumble ingredients, stirring until the mixture forms crumbles. If too wet, add 2–3 tablespoons flour, then sprinkle mixture evenly over the apple filling without pressing down.
Optionally, in a separate bowl, mix crunchy topping ingredients. Sprinkle this mixture evenly over the crumble layer.
Bake for an additional 45–50 minutes or until the top is golden brown and sides bubble.
Remove from oven and let sit for 20–30 minutes to thicken. Serve warm, paired with fresh whipped cream or vanilla bean ice cream.
