Sauté half of the finely chopped onion in a saucepan with the olive oil. Add the wine, let it evaporate, then stir in the rice and toast it for a few minutes. Continue cooking by gradually adding ladles of hot broth. Near the end of cooking, adjust the salt, then remove from the heat and stir in the grated Grana Padano and truffle. Transfer the rice into savarin molds or small timbale molds. Meanwhile, form marble-sized meatballs from the ground beef. In a saucepan, cook them over medium heat with the remaining chopped onion, coarsely chopped mushrooms, parsley, and a bit of roast gravy. Simmer for about 10 minutes. Unmold the rice savarins, spoon the mushroom and meat sauce on top, and serve hot.