In a pan, put the onion (peeled), the carrot, the pumpkin (peeled only if the peel is too thick) and 150g of watercress.
Add the water and boil for 10 minutes.
Add some basil and blend everything.
Put the mixture again in the pan with the remain 50g of watercress (chopped) and let it boil for 5 minutes.
Add the pepper, the vinegar, the salt, and some basil leaves.