Put the crabs or the shells into a large heavy saucepan over high heat, and cook them until the edges of the shells brown a little.
Add two quarts of water, the onion, celery, bay leaf, thyme, and peppercorns. Bring to a boil, then lower to a simmer. Hold at a simmer for 30 minutes.
Strain the stock and discard all the solids. Return the stock to a boil and reduce to two cups of liquid. You can make this ahead and freeze it. (You can also make and freeze more than this quantity for use in other recipes.)
Make a blond roux with the butter and the flour. Add the garlic and sauté until fragrant--about a minute. Add the vermouth and let it come to a boil while stirring it into the roux. After another minute, add the crab stock and bring to a simmer.
Simmer about 15 minutes, stirring occasionally. Add corn and simmer five more minutes. Add cream, green onions, salt, and Tabasco. Stir until smooth and bring back up to a simmer.
Add crabmeat and gently blend in, so as not to break the lumps. Check seasoning and serve hot.