Gratin of chard and new potatoes

Name: Gratin of chard and new potatoes
Ingredients: 5 tbsp olive oil, plus extra for greasing; 450g/1lb ruby or Swiss chard, stems and broad ribs cut into 1cm/½in pieces, leaves finely sliced; salt and freshly ground black pepper; 60g/2oz black olives, stones removed; 2 canned anchovy fillets, chopped; 2 sun-dried tomatoes, chopped; ½ tbsp capers, rinsed and drained; 1 tbsp chopped fresh parsley; 400g/14oz new potatoes, scrubbed, boiled until just tender, drained and sliced; 60g/2oz gruyère cheese, grated; 30g/1oz parmesan, freshly grated