Wash the potatoes and cook them in salted water with their skins on.
Heat oil in a saucepan and fry the onion in it. As soon as it is translucent, add the chopped tomatoes.
When the potatoes are done, remove them from the water, peel them and mash them with a fork. Add to the tomatoes and stir well. Season with salt, pepper and chopped herbs to taste.