1. Prepare and cook the broccoli until just tender. Drain thoroughly.
2. Skin and chop the onion, if used.
3. Melt the fat in a pan and fry the onion until just transparent.
4. Add to the cheese sauce and whisk in the egg yolk; season to taste, and add the nutmeg.
5. Spread a little of the sauce in the bottom of a flameproof dish and arrange the broccoli on top.
6. Pour the remaining sauce over the broccoli and sprinkle with the cheese and breadcrumbs.
7. Cook under the grill until golden; or put into a fairly hot oven, 200°C, Gas 6, for 20 minutes until golden-brown.