Heat the olive oil in a large saucepan over medium heat.
Add the onion and garlic and cook, stirring occasionally, until softened, about 5 minutes.
Add the vegetable stock, potato and watercress and bring to a boil.
Reduce the heat to low and simmer, stirring occasionally, until the potato is tender, about 10 minutes.
Remove from the heat and let cool slightly.
Transfer the soup to a blender and blend until smooth.
Return the soup to the saucepan and stir in the lemon juice, salt, pepper and cream.
Simmer over low heat until heated through, about 5 minutes.
Garnish with the parsley and serve.
Interesting Facts
Watercress is an excellent source of vitamins A and C, as well as calcium, iron, magnesium and potassium.
It is also a good source of dietary fiber and protein.
Watercress is believed to have many health benefits, including reducing inflammation, boosting the immune system, and helping to prevent some types of cancer.