How to make it
Prepare grill for indirect cooking over medium-high heat. (If using charcoal, push hot coals to one side; if gas, leave half the burners off.)
In a large bowl, combine frozen berries, lemon zest and juice, bourbon, allspice, and white sugar.
Make streusel by creaming butter and brown sugar together with a hand or stand mixer until well combined and fluffy. Add flour, salt, and cinnamon and mix again until just combined. (The mixture should be crumbly.)
Use remaining butter to grease a 10-to12-inch skillet or a baking dish. Pour the blackberry mixture into the pan, then scatter the streusel on top.
Place pan on the grill and bake covered 40–45 minutes until the fruit is bubbling and the streusel has browned slightly. Serve.