Pick potatoes of the same size. Place them in a saucepan, cover with cold water, add salt and put the lid on. After the water has come to a boil, cook for about 35-40 minutes or until the potatoes are fork-tender. Check frequently and do not allow potatoes to break. Always start potatoes in cold water to ensure they are cooked evenly.
Drain and set aside to cool to the point when you can handle them. Peel the potatoes and cut them crosswise into slices the thickness of your pinky finger. Do this while the potatoes are still quite hot.
Peel and slice the onions and mix with potato slices.
Season with salt, pepper, olive oil and kvasina (Croatian homemade wine vinegar). Or, substitute kvasina with store-bought wine vinegar. It is important to season the potatoes while they are still warm so they can soak up the flavours.
If you are doing the version with broad beans, cook the beans separately. They take about 15 minutes, and you can even cook them frozen, directly from the freezer. Drain and add to the potato and onion slices. Season and set aside to cool to room temperature.
Wash the arugula and run it through the salad spinner. Add to the salad when it has cooled down. Mix to combine.