In a small bowl, combine the cornstarch and water.
In a cast iron skillet, off the heat, combine the morello cherries, fresh or frozen cherries, and sugar. Place the skillet directly on the grill. Bring to a boil. Simmer for 3 minutes or until the syrup starts to thicken. Add the cornstarch mixture and simmer for 1 minute, stirring constantly with a wooden spoon.
Off the heat, add the alcohol. Flambé, if desired.
Serve with barbecue lemon poppy seed cake and crème fraîche, if desired.