Boil 4 litres of water in a large pot over high heat and add 1 tablespoon of salt.
Put the potatoes in the pot and once it boils back, cook them for about 20 minutes until soft (check with a fork)
Take the potatoes out of the water and let them sit for 10 minutes to cool. Cut in half and place in your serving bowl.
Add the oil, vinegar, onions, capers, olives (if used), sage (if used) and season with the salt and pepper. Gently toss to cover all the potatoes and serve cold.