Preheat oven to 425F.

Cut the potato in half lengthwise, then each in half again (so you have 4 long pieces). Add 1 teaspoon oil and rub all over then place cut side up, on a baking tray. Bake for 30-40 minutes until soft.

Lay the prosciutto over the top of the potatoes then back into the oven for 12-15 minutes until it's starting to crisp.

Set the prosciutto aside to cool (it will crisp more as it cools).

Remove the skin from the potatoes and give them a light mash with a fork. Set aside.

Heat the stock in a medium saucepan over high heat until it starts to simmer. Turn the heat down so that it’s just kept hot but not bubbling.

Heat 1 tablespoon each of oil and half the butter in a large skillet over medium heat.

Add the onion, celery and garlic and sauté, stirring regularly until soft and translucent - about 10 minutes. Keep stirring so it doesn’t colour.

Add the rice, salt and thyme and stir until the rice starts to look translucent. Now stir in the mashed sweet potato until it’s all evenly dispersed.

Add one ladleful of the hot stock and stir the rice slowly until it’s absorbed.

Continue adding the stock, just one ladleful at a time and stirring constantly, making sure it’s absorbed before adding the next. It’s important not to rush this step.

Once all the stock is added, test the rice to see how far from done it is. You can begin adding the water in the same way you added the stock but check for doneness regularly as you may not need it all.

Once the rice is tender but firm and looks silky, take it off the heat and stir in the butter and parmesan.

Top with crumbled prosciutto and more fresh thyme leaves and serve immediately.