Wash the turnips, peel them, but do not cut off the base.
Cook the turnips in boiling water until tender but still firm.
Cut a lid off each one and scoop out the center, leaving the outer part as a cup.
Reserve the pulp.
Melt the butter in a frying pan and crumble the rosemary into it.
Place the turnip cups in the pan and cook for 1 minute.
Preheat the oven to 400°F.
Mash the turnip pulp with the potato and add the grated onion and pepper.
Spoon the filling back into the turnip cases, place in a baking dish and sprinkle with parsley and lemon juice.
Bake for 10 minutes.