Start by placing the slices onions on a tray, cover with boiling water and allow to sit for 10-15 minutes. Drain away the water and pat dry with kitchen paper. Next, place the sugar and 60ml water into a large pan and cook until dissolved, cooking for 5-6 minutes until golden. Add in the sherry or marsala, vinegar, butter and fresh thyme leaves. Next, place in the soaked onion slices, packing them together tightly. Lay over the puff pastry and tuck in the edges. Trim off any excess and place into the oven at 200°C for 30-35 minutes. Take out of the oven and allow to sit for 10 minutes before flipping over onto your serving dish. Sprinkle with coarse salt and serve with the burrata split open on top.