Heat the oil and butter in a large, heavy sauce pan. Saute the onion until tender, about 5 minutes. Add the mushrooms and garlic and continue to saute until the mushrooms are tender and moisture is absorbed, about 10 minutes.
Add the rice and the wine. Stir until the wine is absorbed, about 1 minute. Add broth one cup at a time, stirring the rice continuously after each addition until fully absorbed. Rice is done when tender and creamy, about 25 minutes. Remove from heat and stir in Parmesan cheese. Drizzle with truffle oil.