Heat oil in large skillet set over medium-high heat; cook mushrooms for 3 to 5 minutes or until starting to brown. Stir in shallots, garlic, rosemary, salt and pepper; cook for 2 to 3 minutes or until mushrooms start to brown. Let cool completely.
Place prepared pizza crust on pizza pan or baking sheet. Spread tomato sauce over crust. Sprinkle with mozzarella and 1/4 cup (60 mL) of the Gruyère cheese. Top with mushroom mixture and remaining Gruyère cheese.
Bake according to package directions. Drizzle with truffle oil and sprinkle with chives before serving.