Melt the butter in a stockpot over medium high heat and add the onions, eggplant and tomatoes.  Sauté the vegetables about 5 minutes, until the onions become caramelized.  Whisk the flour into the mixture to make a roux.  Reduce the heat to medium and cook for about 2 minutes, stirring. Slowly add one gallon of water, whisking constantly.  Bring the soup to a low rolling boil and cook for 20 minutes.  Add the shrimp and simmer for 10 minutes.  Season with salt and pepper to taste.