Wash potatoes thoroughly and cook in boiling water for 15-20 minutes
Mix oil and vinegar with a fork. Peel onion and garlic and chop finely. Wash parsley and shake dry. Pluck the leaves from the stems and chop them coarsely. Drain capers and chop coarsely. Add onion, garlic, parsley and capers to oil and vinegar and mix. Season to taste with salt and pepper
Roughly slice the hot potatoes with a fork, pour the dressing over them and swivel
Clean and wash the watercress, shake dry and pluck the leaves from the stalks. Mix the watercress, potatoes and dressing and serve