Plunge the lobster into boiling water and boil for about ten minutes. Drain the lobster and open it using scissors or lobster shears (do not throw the carapace – upper shell – away). At this point, take out all the meat and cut it into bite sized pieces. Melt the butter in a heavy sauté pan, then lightly sauté the lobster meat in it. When the lobster is golden, add the cognac, taking care to keep your face away from any flames that flare up. Over medium high heat, cook the lobster and cognac then remove to a plate or bowl.
Cut the lobster carapace into pieces and put it aside. In a large sauté pan or saucepan, sauté the shallot the olive oil, bay leaves and thyme. Then add the lobster shells and pieces that you have broken up, raise the heat and brown quickly. Sprinkle in the flour, stirring to cook out the raw quality of the flour for, say, 5 to 8 minutes, then add water to just cover. Lower the heat and simmer very gently for about 40 minutes, not letting it boil. Press the mixture through a fine sieve, discard shells etc, and pour into the saucepan, adding the tomato passata and the reserved sautéed lobster meat. Season with salt and pepper, and cook together over medium heat for about 8-10 minutes. Remove from the heat and puree, using either a blender or hand-held stick blender. Finally, add the sour cream, mix well, taste for seasoning, and the lobster bisque is ready to serve!