A good bisque should taste deeply of shellfish and be silky smooth on the tongue. I like to sharpen these flavors by replacing the usual and sometimes plodding heavy cream with sour cream; the slight spike of acidity brings focus to this gregarious mélange. Bisque is a platform that accommodates any shellfish or fish. In place of lobster you might use shrimp, crayfish, steamed mussels, or blue crab with its roe.
Combine the stock, allspice, cloves, and bay leaf, and bring to a boil. Add the lobsters, cover, and cook for 5 minutes. Remove from the heat.
Transfer the lobsters to a bowl to cool. Working over the bowl to catch all the juices, crack the shells to remove the meat; reserve. Return the shells and juices to the pot and simmer gently for 20 minutes. Remove from the heat and pour the fortified stock through a fine-mesh strainer; reserve.
Meanwhile, heat the butter in a separate large pot over medium-high heat. Add the carrots, celery, onion, and rice and cook, stirring occasionally, until the onion is soft, about 7 minutes. Stir in the tomato paste and mace, and cook until the tomato has darkened in color and its aroma has softened, 5 to 7 minutes.
Add the wine, bring to a boil, and boil for 4 minutes. Add the reserved stock, reduce the heat to low, and simmer gently until the rice is fully cooked, about 15 minutes.
Meanwhile, dice half of the reserved lobster meat and reserve for a garnish. Stir the remaining lobster meat into the stock and remove from the heat.
Transfer the soup to a blender, preferably a Vita-Prep, Working in batches, blend, gradually increasing the speed, until the mixture is very smooth and silky (always take care when blending hot liquids). Strain the soup though a fine-mesh strainer, pressing on the solids with the back of a ladle. Discard any solids that do not pass through.
Gently reheat the bisque if necessary. Just before serving, whisk in the sour cream and sherry. Divide the bisque among bowls and garnish with the reserved diced lobster meat. Serve immediately.