Preheat the oven to 350°F (175°C).

In a mixing bowl, combine 4 cups fresh or frozen cherries, pitted, 1 cup granulated sugar, 2 tablespoons cornstarch, and 1 tablespoon lemon juice.

Pour the cherry mixture into a 9-inch pie dish or baking dish and spread it out evenly.

In another bowl, mix together 1 cup all-purpose flour, 1 cup rolled oats, 1/2 cup brown sugar, and 1/2 teaspoon cinnamon.

Pour 1/2 cup melted unsalted butter into the dry mixture and stir until crumbly.

Sprinkle the crumble topping evenly over the cherry filling.

Bake at 350°F (175°C) for 30 minutes, until the topping is golden brown and the cherries are bubbling.

Let the crumble cool for 10 minutes before serving.