In a large skillet, heat the olive oil and butter over medium heat; add the onion and cook, stirring occasionally, until the onion is translucent.
Add the rice and cook, stirring continuously for a few minutes, until the grains are glistening. Increase the heat to high and add the broth and 1 cup of boiling water. Bring the rice to a boil, stirring occasionally. Reduce the heat to low, cover and cook until the rice is al dente, 15 to 20 minutes. Remove the risotto from the heat, stir in the lemon zest and Parmesan and season to taste with salt.