Peel and core the quinces, then cut them into small dice. Place them in a small saucepan with the white wine and sugar, and cook for about 10 minutes.
Once they are tender but still hold their shape (not mushy), drain them, reserving the cooking liquid.
Wash the radicchio and cut it into fine strips.
Place a risotto pan or wide saucepan over medium heat with a drizzle of olive oil. Add the finely chopped onion and gently sauté for a few minutes until softened. Add the rice and toast it in the oil until glossy and translucent. Stir in the sliced radicchio and continue cooking, gradually adding hot vegetable broth as needed, one ladleful at a time.
About 5 minutes before the rice is done, season with salt and pepper to taste. Stir in the reserved quince cooking syrup along with a small portion of the diced cooked quince.
Once the rice is cooked al dente, remove from the heat and plate the risotto. Garnish by distributing the remaining quince pieces over the top. Serve immediately.
