Sweat the 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 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 cream and reheat gently. Sprinkle the chopped eggs on top and serve.