1. Place the drippings and duck skin in a large skillet over medium-high heat. Add 3 cups of the eggplant and sauté until eggplant starts to get soft, translucent and brown, about 5 minutes, stirring frequently.
2. Add the onions and remaining 1 cup eggplant. Cook until the onions start to brown, about 8-10 minutes, stirring occasionally.
3. Add the finely chopped sweet potatoes. Continue cooking and stirring for 4 minutes. Stir in the garlic and cook 3 minutes, stirring occasionally.
4. Add the bay leaves, 1 teaspoon of the salt, 1 teaspoon each of the white and red peppers, the mustard and thyme. Stir well, scraping the pan bottom as needed.
5. Stir in 1 cup of the stock into the vegetables and cook 2 minutes, and then add 1 more cup of stock. Cook 5 minutes, stirring occasionally. Stir in ¼ cup of the sugar and cook 2 minutes, stirring occasionally. Add another 1 cup of stock and cook 10 minutes, stirring occasionally. Add the remaining ¼ cup sugar and 1 cup more stock. Cook 10 minutes, then add another 1 cup of stock and cook 10 minutes more, stirring occasionally. Reduce heat to low and simmer 13 minutes. Stir in another 1 cup stock and simmer for 3 minutes.
6. Remove from heat and strain well, forcing as much liquid as possible through the strainer.
7. Place the strained gravy in a 2 quart saucepan. Add the diced sweet potatoes and 1 cup stock. Bring to a boil over high heat, then reduce heat and simmer 3 minutes, skimming any froth from the surface.
8. Stir in the Grand Marnier and continue simmering for 7 minutes, stirring occasionally. Add the green onions, the remaining ½ teaspoon each of salt, white and red pepper, and the final cup of stock. Bring gravy to a boil and simmer until it reduces to about 3 cups, about 8 minutes, stirring occasionally.