Sauté onions in oil until colorless; add the rice and sauté until transparent. Douse with white wine and allow to evaporate. Gradually add the vegetable stock, stirring constantly; make sure the amount you add each time is no more than the amount which has already evaporated. Caramelize the sugar in a pan, douse with apple juice and bring to the boil briefly. Cut the apple into cubes, add to the pan and bring to the boil. When the risotto is almost finished, add the apple cubes and season with parmesan, thyme and cold butter. Arrange the risotto on 4 plates with a rice cake. Garnish with apple chunks and sprigs of thyme.
Tip: you can also serve the risotto with blue cheese.