1. In a large skillet, heat the vegetable oil over medium heat. Brown the sausages, about 8 minutes. Transfer to a platter with a slotted spoon and set aside. Pour off all but 2 tablespoons of the fat and make a roux by adding the flour, stirring with a whisk or wooden spoon, almost constantly over medium heat until dark golden brown, about 45 minutes. Lower the heat if the roux is cooking too fast.
2. Add the bell peppers and onion. Reduce the heat to low and cook until the vegetables are soft, stirring every once in a while, 6 to 10 minutes. Add a few tablespoons of water to deglaze, the pan, scraping up any browned bits on the bottom.
3. In a medium-size saucepan, bring the 1½ quarts of water to a boil with the bay leaf and salt. Drop the shrimp into the boiling water with their shells on, and boil for 2 minutes. Remove the shrimp with a slotted spoon, reduce the heat to low, and let the water simmer for 15 minutes. Meanwhile, peel the shrimp, throwing the shells and heads back into the simmering water. Strain the shrimp water immediately, discarding the shells and bay leaf. Pour the shrimp stock into a large clean pot and bring to a boil. Stir in the roux mixture, sausage, oyster liquor, tomatoes, thyme, oregano, cayenne, black pepper, Tabasco, and shallots. Reduce the heat to low, and simmer for 2 hours, stirring occasionally.
4. Stir in the shrimp, oysters, and coriander and cook until the shrimp are pink or orange and the edges of the oysters have curled up, 3 to 8 minutes (keep checking). Serve immediately.