Melt the butter in a large pot over medium heat. Stir in the flour, salt, pepper and celery salt until well blended. Gradually stir in the milk so that no lumps form, and then stir in the chicken stock. Cook over low heat, stirring constantly, until the soup begins to thicken. Add the onion and lobster; season with paprika. Cook and stir for 10 more minutes. Stir in the cream, heat through and serve.