Meanwhile, peel and devein the shrimp, combine the shells and water in a saucepan, and reserve the shrimp. Bring the water to a boil, reduce the heat slightly, and cook till the liquid is reduced by half, 15 to 20 minutes. Strain the shrimp stock into a bowl.
In a large skillet, melt the butter over medium heat, add the onion, bell pepper, and garlic and cook, stirring, till softened, 5 to 7 minutes. Add the tomatoes and thyme and cook about 3 minutes longer. Sprinkle the flour over the top and stir well. Add the reserved shrimp and cook, stirring, till they turn pink, about 2 minutes.
Add 1/2 cup of the shrimp stock to the skillet and cook for 3 minutes. Add the tomato paste and stir well. Add the cream, Worcestershire, salt and pepper, Tabasco, and, if necessary, a little more stock to make a sauce that easily coats the shrimp. Keep it on the heat until it's completely warmed through.
To serve, spoon a mound of grits in the center of each serving plate, spoon the shrimp on top, and sprinkle with the parsley.