slice the turnips crosswise 1/8 inch thick. Arrange half of the turnip slices in the prepared baking dish, overlapping them slightly. Pour half of the cream mixture over the turnips and sprinkle 1/4 cup of the grated cheese on top. Repeat with the remaining turnip slices and cream. Cover tightly with foil and bake for about 1 hour and 15 minutes, until the turnips are tender.
Remove the casserole from the oven and uncover. Sprinkle the panko crumbs evenly over the top and bake for about 20 minutes, until the cream is bubbling and the top is golden brown. Let stand for 15 minutes before serving.