Preheat the oven to 220°C/425°F/gas 7 and peel and thinly slice the onion. Place one knob of butter and ½ a tablespoon of olive oil in a medium non-stick frying pan over a high heat. Once hot, add the onions and a pinch of salt, then turn the heat down to low and cook for 20 minutes, stirring occasionally, until softened. Meanwhile, scrub and very thinly slice the potatoes. Unroll the puff pastry and place on a large baking tray lined with baking paper. Use a sharp knife to score a border 2cm in from the edge, then prick the base all over with a fork. Brush with a little olive oil, then arrange the potato slices over the base, so they slightly overlap in one neat, even layer, leaving the border around the edge. Pick the rosemary leaves into the pan of onions and gently mix, then sprinkle over the potatoes. Dice the taleggio into small cubes and scatter over the potatoes and onions and season with a small pinch of salt and pepper. Melt the remaining butter and use to brush the edges of the pastry, then bake in the oven for 20 to 25 minutes, or until bubbling and golden brown around the edges. Delicious served with wilted greens or a crisp salad.