30g butter 1 tbsp olive oil 1 onion, finely chopped
2 leeks, sliced, white part only
2 garlic cloves, sliced 3 floury potatoes, such as king Edward, chopped
1 litre fresh vegetable stock, or 500ml chicken stock mixed with 500ml water 300g
Squeeze lemon juice (optional)
Heat the butter and olive oil in a heavy-based heatproof casserole or large pan over a medium-low heat. When the butter is foaming, add the onion, leeks and garlic and gently cook for 5 minutes or until soft, stirring occasionally.
Add the potatoes and stock, then bring to a simmer. Cover and cook for 15 minutes or until the potatoes are tender. Meanwhile, make the spiced oil. Gently warm the spices in a small frying pan over a medium heat for 1 minute, then add the extra-virgin olive oil and cook for 1 minute or until fragrant. Set aside.
Add 100g of the watercress to the soup and cook for 2 minutes. Remove from the heat, then blend with a stick blender, adding the remaining watercress a handful at a time as you do so. Taste and season; add a little lemon juice if needed.
To serve, ladle into warmed bowls.