In my life, I ate many sides with fish: potatoes, rice, all sorts of vegetables, salad… But the only side I consider appropriate to serve with fish is blitva. This is both the name of the dish, and the name of the featured vegetable, outside of Dalmatia known as Swiss chard. As with many dishes that I have known longer than I have been aware of myself, I cannot be prescriptive with the recipe, only descriptive.
Start with peeling a couple of floury potatoes (I find that having a boyfriend comes particularly handy with such tasks): for every 100 g of Swiss chard, you will need about one medium potato, but this can vary based on your preference for potatoes—in the old days, it varied on the number of mouths to feed, as potatoes were a cheap filler. Cut the potatoes into smaller chunks and boil them in salted water for about 20 min.
In the meantime, prepare your Swiss chard. Wash the leaves thoroughly and separate the white part from the green part. Slice the white parts into 1 cm ribbons. About 10 minutes before the potatoes will be done, add the white parts of the Swiss chard to the cooking water. A few minutes before the end, add the green parts.
Drain the potato and Swiss chard mixture well. It is good to leave the mix in a colander so that it drains well; no one likes a watery blitva. In the pan that blitva was cooking, fry a couple of cloves of garlic with olive oil. Return the blitva to the pan.
Season with salt and pepper and add a generous (and I do mean generous) glugg of olive oil. Mash the potatoes and cut blitva into smaller chunks. When everything is well mixed, it is ready to serve.