Preheat your oven to 350°F (175°C). Grease a 14-inch cast iron or another oven-safe pan with 1 tablespoon of unsalted butter, making sure to coat the surface thoroughly to prevent sticking.

Fill the greased pan evenly with the black raspberries, spreading them out so they cover the bottom of the pan in an even layer.

In a small saucepan, combine the boiling water, white sugar, and the cornstarch that has been dissolved in cold water. Add the remaining unsalted butter (not used for greasing the pan). Place the saucepan over medium heat and cook, stirring occasionally, until the mixture thickens into a syrup, about 5 minutes.

Pour the hot syrup from Step 3 over the black raspberries in the pan. Stir gently to coat all the berries evenly with the syrup, ensuring every berry is glossy and well-mixed. I like to make sure the berries are well coated—this makes the filling extra juicy and delicious after baking.

In a medium-sized mixing bowl, combine the all-purpose flour, packed brown sugar, old-fashioned oats, ground cinnamon, and melted unsalted butter. Use a fork to mix the ingredients together until a crumbly topping forms.

Sprinkle the oat topping evenly over the syrup-coated black raspberries. Place the pan in the preheated oven and bake for 40–45 minutes, or until the topping is golden brown and the fruit is bubbling around the edges.

Remove the pan from the oven and let the black raspberry crisp cool slightly. Serve warm, ideally with a scoop of vanilla ice cream if desired. I like to wait just a few minutes before serving so the juices thicken slightly—this helps get the perfect spoonful with every bite.