Steam the turnips for 15–20 minutes, or until they are soft. Put them into a bowl with a third of the butter, season to taste with the salt and pepper and mash well. Mix the remaining butter with the chopped herbs.
Spread the turnip in the bottom of a warm gratin dish, making a slight hollow in the centre. Cover and keep the turnip warm. Boil the beans in salted water for 4–5 minutes, or until just cooked. Drain well, return the beans to the pan, add the lemon juice and toss well. Spoon the beans into the hollow of mashed turnip in the gratin dish and dot the top with the herb butter.
Mashed swede can be used instead of the turnip, if preferred.