Cut broccoli into pieces and cook in boiling water until tender. Drain in a colander, collecting the water.
Peel and chop onion, sauté in butter until translucent and dust with flour.
Add broccoli water, stirring constantly. Add broccoli to soup, simmer gently, season and strain through a sieve.
Heat again.
Whisk whipping cream with egg yolks and slowly stir into soup.
Arrange the soup and serve.