Combine the lobster meat with the sherry. Cover and refrigerate.
In a hot pan over medium heat, melt the butter.
Once it stops sputtering, add the onion, celery, carrot, salt and pepper and Old Bay. Stir and let cook until the vegetables are translucent and are getting soft.
Add the tomato paste and cook for another couple of minutes.
Add the flour and cook for two to three minutes, stirring all the time.
While whisking, add the cold milk, half and half and stock and bring to a boil. Let boil for about 1 minute, still whisking constantly. Turn the heat down and let simmer until slightly reduced.