In uncovered saucepan, bring water, dried mushrooms, sugar, salt and soy sauce to boil then add rice and reduce heat to simmer. Strain out the liquid and reserve. Chop the mushrooms finely.
In a medium sauté pan melt the butter and add the dried onion and rice. Stir for 1 minute then add the reserved mushroom liquid. Cook uncovered and stir gently until water is absorbed and evaporated, about 15-20 minutes.
Portion rice, drizzle lightly with Parmesan cheese and truffle oil and serve.