Heat the tablespoon of olive oil in a saucepan and add the onion. Cook the onion until soft but not coloured.
Cut the stalks from the watercress, reserving the leaves, and add to the pan with the garlic. Sauté the watercress and garlic for a further one minute.
Add the stock to the pan, bring to the boil and simmer for five minutes.
Remove the pan from the heat, cool slightly and add the watercress leaves. Pour into a blender. Blend until smooth and strain through a sieve. Pour into a serving bowl and set aside to keep warm.
For the poached egg, bring a pan of salted water to the boil and gently swirl the water around the pan to make a whirlpool. Crack the egg into the middle of the whirlpool and reduce to a simmer for four minutes.
Carefully lift the poached egg out of the water with a slotted spoon and float in the centre of the soup.
To serve, drizzle the soup with the extra one teaspoon of olive oil and serve immediately.