Twist off claws and tails from lobsters. 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.