Sweat the chopped onion in the butter for 2 minutes.
Add the watercress and cook, stirring with a wooden spoon, for a further 2 minutes.
Stir in the flour.
Remove from the heat and gradually blend in the vegetable stock.
Return to the heat and bring to the boil, stirring.
Season with salt, pepper, and nutmeg.
Reduce the heat and simmer for 20 minutes.
Blend or process to a purée and return to a clean pan.
Stir in the single cream and reheat gently.
Sprinkle the finely chopped eggs on top and serve the velvety watercress symphony.