In a medium saucepan, bring the broth to a simmer over medium heat. Reduce the heat to low and keep the broth at a bare simmer.

In a large, heavy saucepan, heat the butter and oil over medium heat. When the butter has melted, add the onions and sauté until translucent, about 6 to 8 minutes. Add the rice and coat with the mixture stirring constantly with a wooden spoon.

Add the vodka, stirring constantly until the rice absorbs the liquid, about 3 minutes. Add the lemon juice and continue stirring until the liquid is absorbed. Add the broth, a ladle at a time, stirring constantly. Continue adding until you reach a point where the liquid is absorbed and the rice is creamy and al dente, about 25 minutes.

Fold in the lemon zest and ½ cup of the Parmesan cheese. Season with salt and pepper.

Serve immediately, accompanied by the remaining ½ cup of Parmesan cheese.