Preheat the oven to 180℃ fan/200℃/gas 6.
Prick the sweet potatoes all over with a fork, then tip them into a roasting tin with the quartered onions. Rub the potatoes and onions with the oil and salt, then scatter over the lemon thyme. Transfer to the oven and roast for 55 mins.
For the dressing, mix the chopped basil with the olive oil, garlic, sea salt and lemon juice and set aside.
When the sweet potatoes have had 55 minutes, take the tray out of the oven and cut a cross into each potato – they should be wonderfully soft inside. Lay the slices of taleggio inside the potatoes, then return to the oven for a further 5 minutes for the cheese to melt.
Remove from oven, serve hot, with the onions and dressing alongside.
