- Combine pine nuts, basil, garlic, and lemon zest in small bowl. Season with salt and pepper and a drizzle of oil. Set gremolata aside.
- Heat grill to low. Rub eggplant with a little oil and season with salt and pepper. Grill eggplant with lid down, turning often, until soft and beginning to shrivel, 15-20 minutes.
- Remove eggplant from grill and let cool slightly. Slice an "X" into the top of each eggplant and open like you would a baked potato. Use a fork to mash the inside flesh and make room for the mozzarella.
- Season flesh with salt and pepper and fill each eggplant with cheese. Return to grill. Cover grill and cook until cheese melts, about 5 minutes.
- Transfer eggplant to serving dish. Drizzle with vinegar and spoon gremolata over top before serving.
*Note: Grills vary in temperature, so be sure to look for visual cues, as the cooking time might not be the same. This method of cooking the eggplants whole can also be done in a very hot oven or under the broiler.