Sauté the onion in olive oil for 5 minutes, add potatoes and stock. Simmer with lid on for 15 minutes, leave to cool.
Add the watercress and seasoning, blend until smooth. Store in the fridge for 1 hour.
Toast bread, rub with garlic. Cut into pieces and put in a small pot. Transport soup in a flask and serve with croutons.
Freezing and defrosting guidelines
Make the soup, then leave to cool at room temperature. Freeze (without garnishes or toppings) in a rigid container, leaving a bit of space for expansion, for up to 1-3 months. Reheat either from frozen or defrost in the fridge overnight. Once piping hot, add toppings or garnishes and serve.