Season the poussins with salt and pepper. Place in a large zip-lock bag. Add the vinegar, garlic, onions, and rosemary. Seal completely and place in the refrigerator. Marinate for at least 2 hours, turning occasionally. Preheat the oven to 375 degrees F. Remove poussins from the bag and place on an oval clay baking dish, with a lid. Lay the onions and garlic around the poussin. Add 1/4 cup of water and place in the oven. Bake for 30 minutes. Remove the lid and continue to bake for 30 minutes. Remove from the oven and let sit for 5 minutes before serving.

For the Eggplant and Tomato Gratin: Preheat the oven to 400 degrees F. Grease an 8 or 9-inch porcelain round or oval 2-inch dish with 1 teaspoon of the oil. Season both sides of the eggplant with salt and pepper. In a large saute pan, over medium heat, add 1/2 cup of the oil. When the oil is hot, pan-fry the eggplant until golden on both sides, about 2 to 3 minutes on each side. **Add more oil if needed. Remove from the heat and transfer to the prepared pan. Arrange the slices in 1 layer over the bottom of the dish. Season both sides of the tomatoes with salt and pepper. Arrange the tomatoes in 1 layer over the eggplant. Drizzle the tomatoes with olive oil. In a small mixing bowl, combine the bread crumbs and 2 tablespoons olive oil. Season with salt and pepper. Mix well. Sprinkle the mixture evenly over the tomatoes. Place on the top rack in the oven and cook for 15 minutes. Remove from the oven and let sit for 2 minutes before slicing.

To serve, place each poussin on a serving plate. Serve with a wedge of the Tomato and Eggplant Gratin. Garnish with basil.