1. Purée the grapes in a blender, then strain juices into a bowl. Season with balsamic vinegar if the grapes are too sweet.
2. Combine the flour, cornmeal, salt, pepper and rosemary on a plate, mixing well. Dip the tomato slices in the mixture, coating on both sides.
3. Pour a little olive oil into a large nonstick frying pan, just enough to create a film on the bottom. Add the garlic and place over medium heat.
4. When the oil is hot, add the tomato slices and cook, turning once, until golden on both sides, about 6 minutes.
5. Sprinkle with salt and pepper and, using a spatula, transfer to a serving plate. Pour the grape juices over the hot tomatoes. Serve at once.