Clean the potatoes with a brush, boil them in salt water and pour off. Give the olive oil, the salt, the pepper, and the juice from the pressed out lemon into it. Put the cap onto the pot and shake it. Let it cool and peel the avocados meanwhile. Core the avocados and cut it into thick stripes. Give the potatoes onto a platter by the time they are chilled. Cut the big ones in halves and let the little ones as they are. Put the avocados and the cress onto the potatoes and mix it a little bit.