Cook the new potatoes in salted boiling water until very tender, then drain. Slice the avocado in half and remove the stone. Peel and slice it lengthways into thick slices or chunks (however you really like) and place in a bowl. Slice any large potatoes in half--this will expose their flesh to the olive oil and lemon juice. If they are small, leave them whole. Add to the bowl. Throw the cress in, then add a couple of good lugs (tablespoons) of olive oil and lemon juice, to taste. Season and toss. Serve on a big plate, scattered with any remaining cress.