Prick the sweet potatoes with a sharp knife and put them in a preheated oven, 220°C (425°F), Gas Mark 7, for about 45 minutes or until the flesh is soft when tested with a knife.
Meanwhile, make the caramelized onions. Heat the oil in a large frying pan over a low heat and add all the remaining ingredients. Cook slowly, stirring occasionally, for about 30 minutes until the onions are nut brown and soft.
Remove the potatoes from the oven and put them on a baking sheet. Carefully slice the potatoes in half and pile over the caramelized onions. Top with the sliced taleggio and a sprinkling of thyme and cook under a preheated hot grill for 4–5 minutes until bubbling and beginning to brown.
Garnish with sprigs of parsley and serve immediately with a crisp green salad and a dollop of soured cream, if liked.