Clean the parsnips and cut into slices 1 cm thick. Separately, chop the onion into thin crescents. In a saucepan with a wide arrange bottom half of parsnips, put it on top of the onions and put the remaining parsnips. Pour them with boiling water to cover. Add the oil, pepper, salt, lemon slices and parsley. Stew dish on low heat until the fat remains. Serve it cold.