Cook the sweet potatoes in lightly salted boiling water for 2–3 minutes to soften. Drain and set aside.
Meanwhile, melt the butter in a large, heavy-based saucepan. Add the spring onions and sauté for 1 minute. Add the rice and stir well to coat the grains with the butter.
Add the bay leaves to the rice. Add the hot stock, a large ladleful at a time, stirring until each addition is absorbed into the rice. Continue adding stock in this way, cooking until the rice is creamy but the grains are still firm. This should take about 20 minutes.
Meanwhile, heat 1 tablespoon of the oil in a frying pan and cook the ham until golden. Drain and keep warm. Add the remaining oil and fry the sweet potatoes, turning frequently, for 6–8 minutes, until golden.
Add the herbs to the risotto and season to taste with salt and pepper, then add the ham and sweet potatoes, folding in gently. Cover and leave the risotto to rest for a few minutes before serving.