Preheat the oven to 190 C / 375 F. Lightly grease a 20 cm / 8-inch baking dish with butter or oil.

In a large bowl, toss the berries with the sugar, cornstarch, lemon juice, and lemon zest until coated and no dry cornstarch is visible. Pour into the prepared dish and spread into an even layer.

In a separate bowl, whisk together the oats, flour, brown sugar, cinnamon, and salt.

Pour the melted butter over the dry mix and stir with a fork until it forms small, damp clumps, not a smooth paste. Fold in the pecans if using.

Scatter the topping evenly over the berries, leaving a few small gaps so the filling can bubble through as it bakes.

Bake for 24 to 26 minutes, until the topping is deep golden and the filling bubbles thick and glossy at the edges.

Cool on a wire rack for 5 minutes before serving. This short rest lets the juices thicken instead of running across the plate.

Serve warm on its own, or with vanilla ice cream or a spoon of Greek yogurt.