Remove and discard green tomalley and organs from bodies, reserving shells.
Remove meat from claws and tails, reserving shells. Coarsely chop meat, and transfer to a bowl; cover and chill.
Melt 2 tablespoons butter in a large saucepan over medium heat.
Add lobster shells; cook, stirring occasionally, 6 to 8 minutes or until lightly browned. (The shells add flavor to the broth.)
Add sherry, and cook 2 minutes or until liquid has almost evaporated. Stir in shallot and next 5 ingredients. Cook 3 to 5 minutes or until vegetables are tender; add fish stock and 4 cups water. Cover and bring to a boil; reduce heat to medium-low, and simmer 40 minutes.
Pour mixture through a fine wire-mesh strainer into a large liquid measuring cup, discarding solids. Wipe saucepan clean.
Melt remaining 2 tablespoons butter in saucepan over medium heat.
Whisk in tomato paste and cornstarch; cook, whisking constantly, 2 minutes.
Whisk in broth mixture; bring to a boil, and reduce heat to low. Stir in cream and chopped lobster meat, reserving a few large pieces. Simmer 5 minutes or until lobster meat is thoroughly heated. Top with large lobster pieces, and garnish, if desired.
