25g unsalted butter
1 white onion, coarsely chopped
1 (about 200g) potato, peeled, coarsely chopped
375ml (1 1/2 cups) water
1 chicken stock cube, crumbled (see note)
1/2 bunch watercress, sprigs picked, washed, dried
Heat the butter in a medium saucepan over medium heat until foaming. Add the onion and cook, stirring occasionally, for 5 minutes or until soft. Add the potato and stir to coat.
Add the water and stock cube. Bring to the boil. Reduce heat to low. Simmer, partially covered, for 15-20 minutes or until the potato is soft.
Add the watercress and cook for 5 minutes or until the watercress wilts. Set aside to cool slightly. Transfer the potato mixture to the jug of a blender and blend until smooth.
Transfer the soup to a clean saucepan. Place over medium heat and cook, stirring, for 3 minutes or until heated through. Season with salt and pepper. Pour the soup into a heatproof jug and divide among small serving glasses to serve.