Preheat and prep: Heat the oven to 350°F (175°C). Lightly butter a 9-inch square baking dish or similar 2-quart dish.

Make the cherry filling: In a large bowl, combine cherries, 1/4 cup granulated sugar, cornstarch, lemon zest, lemon juice, vanilla, and a pinch of salt. If using frozen cherries, do not thaw; just add 1 extra teaspoon of cornstarch.

Thicken with flour: Sprinkle in 2 tablespoons flour and toss again until everything is evenly coated and glossy.

Transfer to dish: Pour the cherry mixture into the prepared baking dish and spread into an even layer.

Mix the crumble: In a separate bowl, whisk flour (3/4 cup), oats, brown sugar, remaining 1/4 cup granulated sugar, cinnamon, and 1/4 teaspoon salt. Stir in nuts if using.

Cut in the butter: Cube the cold butter and work it into the dry mixture with a pastry cutter or your fingertips until it forms clumps and sandy bits. You want pea-sized pieces for a crunchy topping.

Top the cherries: Scatter the crumble evenly over the cherries, leaving a few small gaps to let steam escape.

Bake: Set the dish on a baking sheet to catch drips. Bake for 40–50 minutes, until the topping is deeply golden and the filling is bubbling at the edges. Frozen cherries may take closer to 55 minutes.

Rest before serving: Let the crumble sit for at least 20 minutes. This helps the juices thicken so each scoop holds together.

Serve: Enjoy warm with vanilla ice cream, whipped cream, or a dollop of Greek yogurt.