In a bowl, rinse reserved shells well in several changes of water, drain them in a colander.
Combine them with 2 cups water in a saucepan.
Bring to boil and simmer mixture for 20 minutes.
Strain broth through a sieve lined with a dampened paper towel (there should be about 1 ¼ cups of broth, if not, add enough water to measure, or reduce).
In a large skillet, melt 2 tblsps butter over moderately-low heat and in it cook the garlic, stirring, until it is pale golden.
Discard garlic and add shrimp to the oil.
Cook over moderate heat, stirring, for 3 minutes, or until barely pink, but not cooked through.
Transfer shrimp to a bowl, add remaining 2 tblsps of butter to the skillet and cook onion, peppers, mushrooms and celery, stirring, for 5 minutes.
Add paprika and flour and cook over moderately-low heat, stirring, for 3 minutes.
Stir in tomato paste and reserved shrimp broth and bring to boil.
Add shrimp mixture and simmer, stirring occasionally, for 3 minutes, or until shrimp are cooked through.
Stir in sour cream and Tabasco.
Bring mixture to a simmer, stirring, and season with salt and pepper (may be made 1 day in advance and kept covered and chilled. Reheat in skillet over moderately-low heat, stirring, until heated through, but do not let boil.) Spoon Shrimp Creole in center and around edge of rice ring and garnish with parsley.