Score the duck breast using a crosshatch pattern and season both sides with salt and pepper, to taste.
In a large oven proof skillet heat the grapeseed oil, over medium heat, until it begins to smoke, then add the seasoned duck breasts fat side down first. Cook until golden brown on both sides. Remove the duck breasts from the skillet to a baking sheet, and bake for another 10 minutes. Remove the duck to a plate and allow to rest.
Remove all but 1 tablespoon of the duck fat in the same skillet, and over medium heat, add the garlic and chopped onion. Saute until translucent. Remove from the heat and add the brandy. Ignite the brandy with a long kitchen match to burn off the alcohol. Return the skillet to the heat and add the can of cherries. Continue to cook for another 4 minutes, then add the heavy cream, chocolate and the fresh chopped rosemary leaves. Add the cooked duck breasts to the sauce, then remove the pan from the heat and let rest for 5 minutes.
Meanwhile in another skillet, over medium heat, add 1/2 of the butter and a little oil. When hot, add the cabbage and saute until tender. Season with salt and pepper and remove from the heat. Finish the sauce by adding the remaining butter. Whisk well and adjust seasoning, if necessary. To serve, spoon some of the cabbage in the center of each plate and top with a duck breast. Ladle a little of the sauce on top of the duck and serve.