Start this recipe by tossing the diced sweet potato in a baking tray with 1 tbsp oil and season well with salt and pepper. Bake in the oven for 15-20 minutes or cooked through. In a large pan, heat the remaining oil and butter and sauté the onion and garlic for 4 minutes to soften. Add the rice and stir to coat the grains with the butter substitute. Splash in the wine and evaporate. Season with salt and pepper then add in the hot stock, a little at a time. Continue to add the stock mixture, a ladle at a time, stirring every few minutes. Each time, cook until the mixture seems a little dry before adding more of the stock mixture. Continue until the rice is almost cooked through for about 20 minutes. When done, the risotto should be thick and creamy in consistency. Add in the chopped parsley, 2 tbsp of the walnuts, crumble in the blue cheese and add in the baked sweet potato. Stir through then serve the risotto into deep bowls with the remaining walnuts sprinkled on top.