Peel the potatoes and place the skins at the bottom of a medium size pot. Place the peeled potatoes on top of the skins. if the potatoes are not uniform in size, halve or quarter the larger ones as needed. 

Cover with cold water (about 1 cm / 0.5 in above the potatoes) and place over high heat.

Bring to the boil, cover and cook over medium heat for another 30-40 minutes until the potatoes have completely softened: a knife or fork that is inserted into the largest potato comes out easily.

Using a slotted spoon, remove the potatoes from the pot into a large bowl or another pot.

Mash the potatoes with a regular masher or fork until the potatoes are broken. ((or pass through a ricer).

Add 2-3 ladlefuls from the cooking water and the olive oil and continue to work the mash until you get a fine textured puree.

Add salt to taste and more olive oil and cooking water, if you want a less thick mash.

Serve immediately, garnished with chopped green onion.

Unused mash keeps in the fridge 3-4 days, best reheated in the microwave.