Serve this gratin as a first course, as a side dish with grilled chicken, or perhaps just on its own with a fresh tomato salad and good bread on the side.
Preheat the oven to 190C/375F/Gas 5.
Heat two tablespoons of the olive oil in a wide, lidded pan and add the chopped chard ribs. Stir, then cover and cook over a low heat for four minutes, stirring occasionally.
Add the leaves, stir, then cover again and cook for a further five minutes, or until just tender. Season with salt and freshly ground black pepper and set aside.
Place the olives, anchovies, sun-dried tomatoes, capers, parsley and two tablespoons of olive oil into a food processor and blend to make a smooth paste.
Lightly oil a 30cm/12in ovenproof dish. Place a layer of the chard, along with any cooking juices, and a layer of the cooked potatoes into the dish. Spread a little of the olive paste over the chard and potatoes and season with freshly ground black pepper. Continue to layer up the chard and potato, spreading more of the olive paste between each layer and finishing with a layer of chard.
Mix the gruyère and parmesan in a bowl and sprinkle over the top of the gratin. Drizzle over the remaining tablespoon of olive oil.
Transfer to the oven and bake for about 25-30 minutes, until the cheese is browned and bubbling. Serve hot or warm.
