For the radicchio risotto, first cut the radicchio stems in half lengthwise, remove the peel from the root, but leave it on. Soak the halves with water, season with salt, season with pepper and drizzle with olive oil. Leave to marinate for about 15 minutes. Then grill on hot broiler until nice and crispy on both sides. Alternatively, steam in a fireproof dish in a 200 degree oven for 20 minutes.
Sauté the finely chopped onion in half of the butter. Add the roasted radicchio cut into fine strips, finally add the rice, deglaze with wine. Once this has evaporated, cover with clear soup. Simmer quietly over medium heat, keep adding clear soup if the rice needs more liquid.
After about 10 minutes, fold in the finely diced apple. Season with salt and pepper. Simmer on low heat for about 25 minutes, until the rice is cooked but still has bite. Fold the remaining butter and the Parmesan into the radicchio risotto.