To cook leeks: Heat butter and oil in medium-sized saucepan over medium heat until butter melts. Add leeks. Cook for 5 minutes or until softened.
To cook rice: Add rice. Cook, stirring, for 1 minute. Increase heat to high. Stir in wine and mustard. Cook, stirring, for 4 minutes or until wine is absorbed. Add 1 ladle of hot broth. Cook for 5 minutes or until broth is absorbed. Repeat, adding broth and cooking, until rice is al dente, about 20 minutes.
To add cheese: Stir in cheese until it melts. Remove pan from heat, still stirring. Spoon into warm dishes. Garnish with chopped chives.