Chop the green peppers, discarding the central core and seeds. Melt the butter in a pan and cook them gently for 5 minutes. Add the stock and seasoning and cook for a further 10 minutes. Peel and stone the avocados and blend to a purée with the pepper stock. Add the lemon juice and reheat gently; the avocado must not boil. Serve with a garnish of chopped mint.