Peel onions and cook in boiling salted water, uncovered, until almost tender. Drain, and rinse in cold water.
Transfer onions and pan juices to a shallow baking dish; season to taste with salt and freshly ground black pepper. Melt remaining butter in a small saucepan; add flour and stir until smooth. Add cream and cook, stirring constantly, until sauce is thick. Pour sauce over onions; sprinkle with freshly grated Parmesan and bake in a moderate oven (