Put the onion into a heavy-based saucepan. Add the stock and bring to boiling point very slowly so the onion begins to soften a little. Meanwhile coarsely chop a small handful (5 or so tablespoons) of lovage. Add the potato and herbs to the pan and pour on half pint buttermilk. Increase the heat very slightly and bring the mixture to a bare simmer. Stir vigorously all the while as the potato starch will thicken the soup, which may stick to the pan base. Half-cover the pan and reduce heat as low as possible once again. Cook very gently indeed, stirring occasionally, until the potatoes are perfectly tender. Whizz the contents of the pan to a smooth puree. Season with salt and pepper and with extra lovage if you like. Whizz again and thin to taste with a little more stock or buttermilk. Reheat gently or chill thoroughly before serving, then garnish with a swirl of buttermilk and a small flotilla of chopped fresh lovage.